Why would you need this in the US. There are options if you don't like your iPhone.
If you were smart, you would sell your iPhone for $800+ on eBay and buy out of your AT&T contract for $300. $300+$200=$500 leaving you with $300 in your pocket to buy a Droid X or something and go to Verizon. After the $100 rebate you would have $100 in your pocket, unless you get even more for your iPhone since I have seen them go for $1000.
Or, make life easy on yourself and simply return it to Apple/AT&T for a full refund. They extended it 30 days as of about 2 weeks ago so you still have 2 weeks left. You also get out of your contract you signed - so if you were a new customer, no harm done. If you were getting an early upgrade, get something cheap and get away from AT&T or get a different AT&T phone.
All the power is in the buyers hands with this thing, and that extends to the UK. This is marketing drivel.
Of course nobody will be returning their iPhone 4's or doing any of this simply because people aren't that unhappy with their phones. Otherwise there would have been a lot more returns already. Of course, the OP saying "if they did this in the US, that would be awesome" is completely irrelevant as I doubt he owns the iPhone 4 or he would know everything I just said. Anybody who didn't want their iPhone 4 would have returned it already.