According to an article over on pcpro, despite rumour that the iPhone 3GS is exclusive to O2 UK, Play.com are apparently selling unlocked iPhone 3GS units for a staggering £900.
Apparently Play.com ship the 32GB iPhone 3GS without a SIM card and can be used on any network within the UK, and a customer has left a comment on Play.com stating they had no problems connecting to Orange UK.
Going with O2 UK, the 32GB iPhone 3GS costs £274.23 with a monthly contract of £29.38 over 18 months which equates to an ownership cost of £803.07.