Asda have the HTC Evo 3D for £229 (plus £3 standard delivery), see
HTC Evo 3D Android Mobile Phone | Phones | ASDA direct
That is a quite a good deal. However, if you really want the Galaxy S2, I would have thought you could get one in the next couple of months for less than £300, because we are entering a heavy phase of new releases with faster processors. Already, on 24-month contract, there are deals around £20 a month including, say, 200 minutes, lots of texts and 500MB to 1GB of data. Sounds like you only want SIM-free, but look out for a cheap contract with cashback or downgrade rights, which can sometimes be the cheapest way to acquire a phone. Keep looking at HotUKDeals in the Mobile section.