Amazon is looking like your best bet for a SIM free Samsung Galaxy S II at the mo, at £429.49. Looking at the usual suspects, the same 16GB model is £474.99 from Play.com and Expansys, or £478.80 from Clove.
The phone is sold direct through Amazon (as opposed to a third-party seller), and the Samsung Galaxy S II is delivered for free.
The Samsung Galaxy S II has a 4.3in Super AMOLED display, with a dual-core 1.2GHz processor and 8MP camera. (full specs)
(Thanks to scoopsy at HUKD)