Capt D
If you haven't got easy access to the UK AIP and ANO etc, get a copy of the UK AIM - very handy little book that fits in your flight bag. Only costs £14.95.
According to the note at the bottom of AIP AD 1.1 11, it would seem you can land at any aerodrome outside controlled airspace, without regard to any minima! This does not answer your question but, going back a few pages, AD 1.1 seems to give some answers. There is a handy table in AD 1.1 4.1.2 which gives the take-off RVR's for "performance class A" aircraft.
AD 1.1 4.4 gives the single pilot RVR of 800m for approaches - this can be reduced (to the "normal" minima) if you have an ILS-coupled autopilot.
I am fairly sure that if you have the ILS-coupled autopilot in your "Performance Class A" light twin and you are single crew - it is possible to depart with RVR as low as 150m (subject to a suitable take-off alternate being available).
Please put me right if I'm wrong on that last point - interpretation of these rules can be a pain!!
Cheers,
ETR