Apple iPhone 3GS mat swap for a HTC desire S only ?

im looking to replace this with a HTC Desire S, so do not need it anymore, I believe it is locked to O2, ive tried a Giff Gaff sim and it works fine, but giff gaff uses the O2 network, in its life it has had the front screen replaced, there are some usage marks on the phone, but the screen is unscratched as it has had a protector on it, comes with charger and all in the pic collection from London or Herts Watford any questions please ask or if you need this sooner please contact me and i will list it with a buy it now