The official answer is in ENR 1.1 of the AIP.
http://www.ead.eurocontrol.int/eadba...2013-12-12.pdf
Not sure how long that link will work for.
The essence is:
A VFR flight experiencing communication failure shall:
When VMC can be maintained, the pilot should set transponder on Mode A, Code 7600 with Mode C and land at the nearest
suitable aerodrome. Pilots should take account of visual landing aids and keep watch for instructions as may be issued by visual
signals from the ground. The pilot should report arrival to the appropriate ATC unit as soon as possible. When VMC cannot be
maintained, the pilot should adopt the procedures for IMC detailed below.
That leaves a lot open to the pilot to decide. What is a suitable aerodrome? It will obviously change quite considerably depending on your aircraft, fuel level, weather, any other operational emergencies.