there are laws. ive forgotten what the parameters were exactly but a ballpark (although a ballpark would be too small to fly this in. haha!) would be something like 50kg max takeoff weight and maybe some limit on horsepower, again maybe 50hp (but im sure ive seen more powerful with those guys with heaps of cash to burn who fly giant scale reno-racers).
edit:
from the site:
Over 20 Kg Scheme
For models weighing over 20kgs the LMA operate a scheme on behalf of the Civil Aviation Authority. For the Bulldog, John Townsend who was the LMA Safety Officer for many years, acted as the inspector. Ian discussed with John the construction and various details and John inspected the basic airframe at one of the LMA meetings. John made a second final inspection when the model was completed and in due course Ian received an exemption certification to allow flight tests.
and:
Flight Trials
To gain an exemption certificate, which allows you to fly an over 20Kg model at public events, it's necessary to complete a flight test programme. A flight log sheet is required to be completed. These flights have to be witnessed by a person authorised by the Large Model Association. Ken Wooten, the owner/operator of the airfield near Withernsea, East Yorkshire, was asked to witness the flights.