Years ago I was told of a location in Lundun (innit?) near the Oz Embassy where Ozzies who were due to return home parked campers and waited for Ozzies arriving on a tour of the UK, mostly young gap year students. The idea was to buy & sell campers, which sort of worked on an unregulated basis. Vehicles probably rather old.

There is the question of taxing the vehicle and getting an MOT, not to mention insurance (before taxing, though the insurance may be invalid if the MOT &/or tax are not current).
Not having done it from an indeterminate address I do wonder how it is done. On-line obviously, but where do you claim as base? The Oz embassy? Vehicle insurance is dependent on "location", and nationality!. Lundun can make it more expensive.

Anyway visits to clubs, sessions & festivals in my area (look for "map") can be planned using mister.red though the datestamp should alert you to ring ahead.

For other areas I do a list of listing sites and with luck there will be others who will post their lists here. RoW (rest of world)