The CAA website is a bit like the UK's approach to signposting... there's never one when you really need it.
The information is in LASORS (
http://www.caa.co.uk/docs/33/LASORS_06_WEB.pdf), specifically section A.7.
Providing that your certificate and medical are valid, then if you are flying a N reg in the UK, you are flying with the full privileges of the FAA certificate. If you wish to fly G reg, you are restricted to VFR.