HI yes you can use sony ericsson equinox mobile with uk o2 network sim only if it is unlocked one ...Else if the phone is locked to network other than o2 then until you unlock it you can't use it with any other GSM sim ..You can easily unlock your phone using sites like WickedUnlock.com they are providing unlock code for all mobile models at reliable cost ...After unlocking it you can freely use any GSM sim in it .