With respect to buying a new iPhone – are you OK with one that is carrier-locked, or do you want one that is not locked to any carrier?
My understanding is that in the U.S., phones sold by Apple Authorized Resellers are usually carrier-locked. Even when they start out in a "flex lock" state, they lock themselves to the first carrier that you use. If you are looking for an unlocked phone, the way to get one is to buy the phone directly from Apple themselves – avoiding any buying option that involves carrier financing.
I don't know how it works in the U.K., but I'm guessing that it might work in a similar fashion – that you might have to go to extra trouble if you want a phone that you can use with any carrier.