The biggest three carriers in the UK now have the Samsung Galaxy SIII available for pre-order. Here's a quote with some details,

you can snag the 16GB model at no cost with a 41 monthly plan on Vodafone. Of course, O2 and Three offer similar plans, with others ranging as high as 219 for the handset but with less of a commitment.
Thanks for the tip, RobKort!

Source: Engadget