MAM of trailer must not exceed MAM of tow vehicle, or its rated towing capacity.
Trailers of >750kg MAM must be braked. Overrun brakes max out at 3500kg. Brakes must operate on all road wheels.
Combined MAM of vehicle and trailer must not exceed 3500kg, except where the trailer is <750kg MAM unbraked, in which case the combined MAMs must not exceed 4250kg.
A-frames/tow dollies are not legal under almost all circumstances. They are classed as trailers, with the same MAM as the vehicle being towed, and as such almost always need to be braked, on all wheels. The AA etc get away with this as there is a "to a place of safety" exemption for recovery work.
This means many of those large 4x4s with a large caravan and younger family may well be breaking the law. eg my disco 300tdi could only tow a braked trailer of 780kg unless the driver has +E
On the subject of Sankeys, the military designation has two types, 1/2 ton & 3/4 ton. This would the be permissable design payload for each variant. So the 3/4 (762kg) ton version definitely not allowed without a +E on larger vehicles, the 1/2 ton (509kg) it would depend on the unladen weight, it would have to be less than 241kg unladen. (Being old army, thats old tons, not new 1000kg tonnes. Off to the weighbridge with empty sankeys then everyone