The iPhone 8 is available to pre-order from today, but don’t be distracted by flashy offers with low upfront costs and a high monthly fee: the cheapest way to get one is still simply buying it outright from Apple or another retailer, and taking out a low cost sim-only contract.

The 64GB iPhone 8, the cheapest of the newly launched phones, costs £699 when bought directly from Apple or from a third-party electronics store such as Currys or John Lewis. Combined with a low-cost contract or pay-as-you-go sim, such as the £5 plan offered by O2’s corporate sibling Giffgaff, the cost of owning the phone for two years is £819 – lower than any competing deal of the major retailers we reviewed.

Only one mobile phone operator deal came close to offering better value for money: Sky Mobile charges an up-front fee of £99, with a monthly cost of £37 for the first 12 months and £28 for the second 12 months – a total outlay of £879. But the Sky Mobile deal, with its Swap12 plan, lets you trade up to a new phone 12 months in, restarting the cycle (and locking you into a contract for another two years).

The Sky Mobile deal means you are, in effect, paying £543 for the iPhone 8 over the one-year period, a saving on the £759 you’ll have to pay for the same period if you pay up-front and take out a low-cost sim deal for a year. But unlike buying the phone in one lump sum, you don’t get to keep your iPhone when you decide to trade it in.

That means the distinction largely comes down to two questions: how happy you are navigating eBay or similar, where a one-year-old unlocked iPhone will typically sell for at least half its new value; and how much you want to avoid being locked in to one mobile operator for the long term (Sky Mobile uses O2’s network, so its coverage will be the same).

Facebook Twitter Pinterest The iPhone 8, iPhone X and iPhone 8 Plus are displayed at Apple’s launch event in California. Photograph: Justin Sullivan/Getty Images

Even if your main interest isn’t to keep the price as low as possible, the most cost-effective way to get a new iPhone is still to buy it outright from Apple and then pick up a sim-only contract that carries the best mixture of minutes, SMS messages and data to suit your needs. A word of warning for the worst plans we found, which are O2’s “deals” that let you “pay for your device in full”: the company charges £771.99 up front for a phone that Apple sells for £699, and still locks you in to a two-year contract on tariffs that start at £19 monthly.

Finally, it’s worth mentioning the iPhone Upgrade Plan. The company splits the cost of an iPhone and AppleCare+, its premium warranty service, over 24 monthly increments, but allows customers to upgrade to a new phone after 11 months. As with Sky Mobile’s Swap12 plan, the deal can be worth it if you intend to get a new phone every year, but it comes with similar lock in: if you decide to switch to Android, you’ll be stuck paying off your iPhone for a full two years. The forced AppleCare+, meanwhile, will be a waste of money for most users – unless you smash your phone’s screen, in which case it vastly reduces the cost of fixing it.

And one word of warning: the iPhone 8 is not the iPhone X, Apple’s all-singing, all-dancing, all-screen “future”. The 8 will start shipping 22 September, while the X is coming on November 3. If you’re in two minds about which model to go for, it may be worth waiting another couple of months until the first set of reviews come in for that device before tying yourself down to an iPhone 8.

Indeed, new information is still coming through about the iPhone X. Apple executive Craig Federighi just yesterday revealed some more news about how the device’s Face ID system works, in an email to an Apple developer. The feature uses IR light, which shines through “most sunglasses”, he said, meaning it will still work on your beach holiday (unless you are unlucky); and an emergency lock feature, activated by “gripping the buttons on both sides of the phone”, prevents it being simply stolen and unlocked coercively by a thief.

Whether Face ID really works as well as Apple says it does remains to be seen.