Just to update people on Three. I went to the Durham store this morning, no reservation or anything, and they were very happy to give me a PAYG 4S. They said they were not allowed to sell it without the £15 though. So add £15 to the purchase cost.
Once you've bought it you have to activate it, which I guess you could do with another 3 sim if you wanted to keep the £15 one to sell. Then ring 333 and ask for it to be unlocked. That's another £15 and, according to the CS I spoke to, it takes 24-48 hours so he says mine will not be unlocked until Monday as it is Apple that do it and they don't work Saturdays.
So, the best bet is to get one direct and unlocked from Apple. But, if you're willing to pay £30 to get one sooner, try Three.
+1
I placed an order with Mazuma just after ip4s came out, so if i can get my ip4 back to them in time i get £270 for it, whereas on the website now its only paying £210 odd. So im still up, after buying from 3 (which i did, very easily today)