The most striking thing about meeting Rob Halford is the sheer disparity between the way he looks and the way he sounds. The man whose fans call him the Metal God – a title he has trademarked – is clearly one of that select breed of rock star who's never off duty, at least sartorially. Opening the door of his Las Vegas hotel suite (he's there to mentor amateur musicians in something called a Rock and Roll Fantasy Camp) he looks much the same as he does on stage, fronting Judas Priest: a 6ft figure, clad entirely in black, his long goatee beard dyed to match, his head shaved and tattooed, his eyes hidden behind a pair of aviator sunglasses. For a self-proclaimed "person of faith", he looks pretty mephistophelian. And then he opens his mouth and, I'm afraid, the spell of diabolic menace is shattered in an instant. He has a lovely, gentle voice: furthermore, his accent is still firmly resident in Walsall, years after its owner relocated to America.

It's tempting to say Halford's voice suits him perfectly. Before I meet him, I'm furnished with a lot of advice on how to approach him by his record company. It's clearly meant to be helpful, and it probably tells you less about Rob Halford than it does about the metal scene's understandable prickliness towards the mainstream media, which has spent decades sneering at and mocking it. Nevertheless, it has the effect of making me expect him to be difficult, and Halford, it quickly transpires, is about the most delightful, down-to-earth Metal God you could wish to meet: "Oh, I've never gone off into that 'the room's not the right temperature, take this tea back' stuff," he frowns. "I still scrub my own toilet and vacuum the carpet, and I have to be able to push my trolley around Morrisons and do my shopping."

Judas Priest in their heyday... from left to right: Glenn Tipton, Rob Halford, KK Downing, Ian Hill

He is also wryly funny about everything from the brief period when he quit Judas Priest in the 90s ("my mid-life crisis, I call it") to his sexuality. When I mention that Judas Priest seem to have become a beloved institution – a state of affairs far removed from the kind of consternation they caused in the 80s, when they were variously accused of entreating their fans to commit suicide via subliminal messages and listed at No 3 in the PMRC's "Filthy Fifteen" list of objectionable artists – he nods: "I feel a bit like Quentin Crisp these days, I've become the stately homo of heavy metal." Indeed, his sexuality was a topic the record company advised me not to dwell on, but Halford brings it up before I do, cheerily suggesting that coming out during an MTV interview in 1998 was less an act of defiant bravery than a slip of the tongue. "I just say what's rattling out of my brain, you know, and I just happened to go, 'Well, speaking as a gay man …' and then I heard this noise, and it turned out the producer had literally dropped his clipboard when I said it."

Slip of the tongue or not, he says, it was "the greatest thing I could have done for myself". Now he wonders why he didn't do it sooner. "I think I just built in this delusional fear that I was going to destroy myself, no one was going to look at me any more as a metal singer, I'm going to destroy Priest because of my attachment with them. It was all self-imposed paranoia. It didn't affect Priest one iota: the record sales didn't plunge, the show attendance didn't plunge. Unconditional love will accept you for who you are, and I think that was the blessing I had from the fans."

We're supposed to be discussing Judas Priest's 17th studio album, Redeemer of Souls, but we're rather hampered by the fact that, because "there used to be this thing called rock'n'roll magic, when things wouldn't leak, and we're trying to keep a little bit of that going as best we can," Halford isn't allowed to say much about it, beyond the fact that it "reasserts the classic Priest sound" following 2008's double concept album Nostradamus, which met with a mixed reaction from fans. He says he doesn't really know what guitarist Glenn Tipton meant when he suggested that it was Priest's "farewell album, although it might not be our last". He shrugs. "It's a funny thing, isn't it? The last tour we did was billed as a farewell tour and we're still playing live. What we were trying to say is, we're winding down, we're not going to go out on these big two or three year schleps any more, because" – he gives a knowingly dramatic pause – "heavy metal is immortal, but we're not."

The concept of a farewell album that may not actually be the artist's last is undoubtedly a slightly peculiar one, but, in fairness, Judas Priest have always been a slightly peculiar band. The other great architects of metal in the 60s and 70s arrived more or less fully formed: they changed and developed over time, but drop the needle on the first track of Led Zeppelin or Black Sabbath's eponymous debut albums and you get the general idea of what the band would be. Judas Priest, by contrast, were all over the place: online you can find a clip of them on Whistle Test in 1975, with Halford resplendent in something that looks like a cross between a kaftan and a kimono - or as he puts it, "looking like I don't know what" - playing an odd, ungainly hybrid of prog, blues-rock and folky wispiness. The response was tepid – Halford claims that one review of their second album, 1976's Sad Wings of Destiny found the writer hoping aloud that they still had day jobs, "because this music isn't going anywhere" – and they didn't really hit on their signature sound until 1978's Stained Class and Killing Machine. By then Halford had come up with the band's leather-and-studs look, famously borrowed from the gay S&M scene: "it's funny, because I've never been into all that stuff, quite frankly. Forget your sling, you know: it's never appealed to me. I don't think in all honesty I've ever put anything from my own sexual side into my life with Priest because, much like everybody else, it's private."

Astonishingly, no one straight seemed to cotton on to the look's connotations – "of course, my gay friends were immediately like, 'What's all this about?'" – but, then again, Halford says, people have always overlooked stuff about Judas Priest, not least the political slant to at least some of their lyrics. In his telling, Breaking the Law, from 1980's masterly British Steel, was a kind of metal counterpart to Ghost Town, inspired by the winter of discontent and the rise of Margaret Thatcher. "All of that's in there, you know: 'Completely wasting, out of work and down' – no one cares, I'm going to break the law. We weren't giving people affirmation to break the law, but we could understand their frustration."

They were also more musically restless than the popular image of them suggests. They experimented with synthesizers to yells of horror on 1986's Turbo, and a couple of years later went into the studio with producing trio Stock, Aitken and Waterman. This latter is widely held up as evidence of how lost Judas Priest were in the years immediately after their commercial heyday, but Halford is winningly unrepentant. "Oh, I was a huge Rick Astley and Bananarama fan. I remember saying at the time, 'You're probably going to think I've lost my fucking mind, but what if we did something with Stock, Aitken and Waterman?' And the rest of the band went, 'Oh, that might be interesting.' You know what song we did? You Are Everything, the Stylistics song. A Priest version of that! It's absolutely stellar! It totally worked! I was dead chuffed, but in my heart, I thought, 'This is never going to be released,' because there would have been a huge kickback."

At 63, recovering from a back injury that he thinks may have had something to do with taking to the stage "wearing literally 30 pounds of leather and steel spikes" night after night, he seems to have lost none of his enthusiasm for the genre he helped create: "We haven't become jaded or cynical, we love what we do, we have a great relationship with each other and the metal just keeps on coming." In fact, he talks about heavy metal with a kind of fervour that seems vaguely evangelical, in every sense of the phrase: "It doesn't matter whether you're a beggar or a king, metal will find it's way into your heart if you accept it."

Still, he says, being in Vegas has turned his thoughts to unfulfilled musical goals outside of "my metal heart". Perhaps unexpectedly, given that he describes his own vocal style as "screaming your tits off for two hours a night", he is a huge fan of Michael Bublé. "I've said before that by the time I'm 70, you'll find me in a little joint just off the Vegas Strip and I'll be going" – he bursts into crooner-style singing – "'breakin' the law, breakin' the law'." He hoots with laughter. "Remember this one, folks? 'Livin' after midnight, rocking to the dawn …' I'll be drinking and smoking again, which I haven't done for 30 years, and I'll be leaning on the bar stool, like, you know, whatshername out of EastEnders. I've said it jokingly." He frowns. "Or is it jokingly? Stepping outside of my comfort zone … I think maybe, as I'm in my senior years I don't give a fuck now, whereas maybe I used to."

Redeemer of Souls is released on Columbia on 14 July