Cheapest way to buy an iPhone in the UK / The seperation of devices and carriers

I have suggested this to everyone I know who asks me about getting an iPhone on contract.

I used to send out a little excel sheet detailing the expected costs and actual costs to show how much differecne there was, especially as most UK contracts now want you to sign up for a 24 month deal, tying you in to a longer contract at a rate of £30 or £40 per month.

I currently use O2 but will be switching to giffgaff next month because of that unlimited data deal.