Have a look at http://giffgaff.com/%5B/url%5D and consider buying the phone separately and unlocked if you think you’ll have it for a while. I ended up sticking to my contract as it worked out even once they’d decided they would give me the one x for no extra cost.
I’m enjoying the one x, everything is slightly better than the desire hd I had previously with the screen, camera and outright processing speed being the main improvements. It is a large phone, fortunately I’ve got farmer’s hands so it suits me well, but due to that lovely big screen it is power hungry so if you’re a heavy user maybe consider getting something smaller. It isn’t a major upgrade and if I’m honest I didn’t really need it as the desire hd was perfectly adequate. Next time I think I’ll go sim only as in my opinion paying for a phone in one cash lump makes the decision more considered.
I’d recommend the one x, but the one s is also supposed to be very good and I think the battery might last slightly longer. Maybe hold off for two months as the galaxy s3 will be out, but then there is always something around the corner.