In the UK, in virtually all cases, it is illegal for minibikes of 50cc to be ridden on the road. Minibikes that are 45cc are legal to be ridden in public fields, although the public areas must be at least 20 meters away from houses, as then they become illegal due to the noise. Minibikes 50cc or over are regarded as "Motor Vehicle" as defined by section 185 of the Road Traffic Act 1988 and so to be ridden on the public road, a "Motor Vehicle" must comply with all aspects of road traffic law and meet the mandatory European construction requirements by having a "Certificate of Conformity" indicating that they have European Community Whole Vehicle Type Approval (ECWVTA).
Minibikes are not manufactured to meet these requirements, and would need to be modified significantly to do so. Reported instances of this occurring are incredibly rare, meaning that it is very unlikely that minibikes can actually be registered for road use.
(above statement sourced from Wikipedia)
However that said... There are statements that differ and I believe the Law is Unclear... Some say you can only ride on 'Private Land' with the Land Owners Permission etc, but Maybe the Upper Statement is Closer to the Real Law, Anyway the powers that be - have deemed these 'toys' as being an anti-social nuisance and with 4 deaths being highly profiled, they will do their utmost to put a stop to any complaint made against you and stop you in your tracks from having any innocent fun which is what these mini bikes where intended for in the first place. The Law itself is very unclear as most police forces will treat them as a "motor vehicle" which of course they aint.. Ride it on a Public Highway or Pavement etc and your goose is cooked if you get caught, so *DONT*
All I can really advise is take it to a disused building site and if the cops try to ask what your doing, all you say is the land is private and you have permission and inform them that they are the ones actually trespassing lol.