As well as the options given already, opening your flight plan can be done with London Information (freq on chart).
And don't forget that anywhere outside the UK, ATC expects you to close your flight plan as well, or the rescue services will come lookin'.
Closing is done automatically upon arrival at a controlled airfield. If you fly to an uncontrolled field, either close it with the last ATC facility you're talking to, or phone up a FIS after landing.