I'm not so sure why you wait that long, maybe things are different in the UK, but I think this info could be useful to a lot of people. I'll just throw out a scenario I tell everyone when it comes to buying Apple devices that's strictly just financial and not exactly being an Apple 'fanboy.'
Two options here, if you buy a phone on contract for say $200, then you can always sell it in a year, or even two, for around double of what you paid for it. Example, iPhone 5's are going for around $450 in my local market. I just paid $200 a year ago for it on contract. Some people hate contracts and say it's cheaper to not have data or what have you, but like I said this is just this particular scenario if you are on a plan.
Option B, you can buy a device for full price, ie $650 and wait the year or two and sell it for ~$450 but now you have a gap of $200, but you still own the device outright. Sometimes depending on what carrier, you can get an iPhone discount of $100 when not in your contract and that could bring it down to ~$550.
These examples exclude taxes and exclude some abnormalities with the pricing and buying markets such as unlocked > locked phones such as Verizon. Verizon phones don't have the same desire as AT&T phones because of the CDMA vs GSM difference with being able to use it on other prepaid carriers easily and worldwide use.
Anyway, that's my two sense from what I've learned over the years. If you buy a phone on contract and take care of it, you'll still get a great phone and double your money in a year when you go to sell it. That's how the markets have been treating them so far over the years.
Edit: I will say that if you wait too long such as the large jump from 4 to 5S, the resale value on the 4 is now dipped to ~150-200. So to me it makes more sense buying every release or two on contract, and then letting the phone pay for itself.