It was hardly the most evocative of locations – an empty, overlit office space with on the 17th floor of ugly concrete towerblock that is home to Westminster Council – but when the moment came, with the late afternoon sunshine casting the panelled ceiling and carpet tiles in a luminous gold, Percy Steven confessed to a flicker of nerves and an unexpected lump in his throat.

Forty-eight years and 10 months after they first became romantically involved, Steven, who is now 75, and his partner Roger Lockyer, 87, on Wednesday became one of the first couples in the country to convert their civil partnership to a marriage, allowing the registrar to declare them, to broad beams, “husband and husband”.

For the first year of their relationship, gay sex was illegal and they could have been jailed just for being together. Nine years ago they became one of the first same sex couples in Britain to register for a civil partnership. Now, at last, they could refer to themselves without hesitation as married and it felt, said Lockyer, “really rather lovely”.

They were not the very first – that honour, in Westminster, fell to the TV presenter and fashion consultant Mary Portas and her partner Melanie Rickey, for whom the council opened their offices especially to allow for a quick ceremony at one minute past midnight.

Portas had arranged the event as a surprise for Rickey, more than four years after the pair legally became partners. “I thought we were out looking for art galleries but when we started driving up and down the same street a couple of times I started getting a bit suspicious,” said Rickey. “When we turned up outside Westminster City Hall I guessed what was going on.”

“We have three children together and there is such a deep rooted commitment to being married, it just felt right,” said Portas.

The option to “convert” a civil partnership to a marriage, made possible by an amendment to the Marriage (Same Sex) Act, resolves one anomaly to replace it with another. The process brings no change in legal status, at least in the UK, but gay couples who became legal partners before March, when marriage was extended to all, now have the option to remain as civil partners or go through a brief ceremony in order to call themselves officially married. No such choice is open to heterosexual couples however, for whom, for now at least, marriage remains the only option.

Nick Clegg said the change in the law represented another important landmark for LGBT rights, but was also a reminder of those across the world facing discrimination because of sexual orientation. “So as we raise a glass to those making their vows, we send a message of solidarity to LGBT people everywhere.”

Having become civil partners in 2005, Steven and Lockyer had felt no great need to change their marital status again, but after the idea was suggested, the more they considered it, “we just thought, why not?” said Lockyer, a retired university professor. How did they think being married would be different? “I don’t know,” he said after a pause. “To me, it feels like reaching the top. We really are married. Which is what we have been for nearly half a century of course. But to have formal recognition of that I think is something that’s so unexpected and wonderful.”

Steven, a theatre director before his retirement, joked that he was looking forward to describing himself to utility call handlers as the husband of the bill holder. “When I say we are civil partners that doesn’t seem to make much difference. Now I can ask for an account in both names.”

As befitted a change that as well as being enormously significant was largely administrative, the conversion ceremony, one of 25 carried out in Westminster on Wednesday , was brisk. Passports and their civil partnership certificate were produced from a folder, their names and address written carefully on their marriage certificate, along with the names and occupations of their fathers, proving that equality has not yet been reached in every aspect of the marriage ceremony.

A signature from each and the deed was done. “I feel different already,” said Lockyer with a grin. Steven picked up their champagne flutes and moved to join him, tenderly pausing to flatten his new husband’s hair before they posed for photographs.