This is copied from another post I typed about 18 months ago. Rules are still the same as far as I know;
Theres a lot of crap been posted here as "Fact" by people who evidely have no idea what their on about. Can I please clear stuff up once and for all. This is as I see the rules and regulations, and it is most peoples interpretation of them. It is not legal advice, however I belive it to be correct.
At 16 you may take a tractor on the road with L plates. It must have no trailer. It can only be driven to and from a class F driving test, on any road where tractors are legally allowed to go (ie; not motorways.) It must be less than 2.45 meters wide at the widest point.
Once you have passed your class F you can drive any tractor PROVIDED it is less than 2.45 meters wide. You may tow a trailer PROVIDED it has 1 or 2 close coupled axels. (so no artics), and it also has to be less than 2.45 meters wide. Any implement can be attached PROVIDED it is less than 2.45 meters wide. An implement means that it has no wheels on the floor while on the road.
At 17 you may take your class B driving test (car) you may also (if not already done the class F test) take any tractor and trailer combination on the road PROVIDED it has L plates. L plates are not needed once you pass class B test. You can tow any trailer behind a tractor as long as the Maximum Autherised Mass (MAM) is less than 24 tonnes, whether you have passed your class B or not.
Once you have passed your class B test then the tractor does not need L plates but you cannot drive certain machinary (combines, sprayers etc) until you are either 18 or 21 (depends on the machine)
You may also tow a trailer maxium LOADED weight of 750kg behind a car if it has no brakes. You are also allowed to tow a trailer of any weight LESS than that of the towing vehicle as as long as the OUTFIT (ie car and loaded trailer) weighs less than 3.5 tonnes. Eg if you had a 2 tonne LR then you could tow a 1.5 tonne trailer, as long as it is braked. You wouldnt be allowed to tow a 2 tonne trailer (MAM > 3.5t). Equally you wouldnt be allowed to tow a 1.5 tonne trailer behind a 1 tonne car.
To tow more than the MAM of the car, you need a class B + E licence, which involves a basic reverse and genearal road driving. It costs I belive about £500 with training.
You cannot take tracklaying tractors on the road until you have passed a class H test. The exception being a Quadtrack because it is pivot (artic) steering as opposed to steering with the tracks. You need to be 21 to do the class H test I believe.
As far as getting started, apply for a provisional driving licence and once youve got it book a class F test. You can do this online on the DSA website. www.dsa.gov.uk
www.direct.gov.uk for more info, and the full guidelines.
Saying that "no-one will police" a law is a false economy. You have an accident and are too young or not licenced properlly then your insurace will be cancelled and you will face legal proceedings from the police as well as having to pay for any damage out of your own pocket.