My better half and I have been using e2save for the last probably 6 years. You pay up front for the contract and get cashback by cheque (redemption) which covers some or most of the contract cost. My partner got a HTC Wildfire S which does everything we need, with unlimited data on T-mobile for around £6 per month after cashback, but that was 18 months ago.
The only risk with cashback websites is if they go under / disappear as you are still contracted in to pay the provider the full fee for the length of the contract, but e2save are part of Carphone Warehouse and we have successfully claimed cashback for years. Just need to remember to send off the original phone bill every three months and get a cheque through the post.
May or may not have an offer for the phone you want, but worth searching around online. If you definately want the Galaxy S II then just search for that… T-mobile seem to do mostly unlimited data.