Max towing weight for a landy 3.5 tonnes - 4 tonne with coupled brakes.
How much can you tow legally? Difficult question to answer because the rules are different between 'for work use' or 'private'
Basically if you are towing for yourself.
Your Maximum gross weigh (MGW) can be up to 7.5 tonne, any distance, any time and for as long as you like. IE MGW (vehicle, including occupants + overall weight of trailer including weight of goods in trailer)
If your towing for work:
MGW is 3.5 tonnes overall, IE, if vehicle and trailer together are above this weight, you must fit a tacho. Meaning, restricted working hours of driving, cost of tacho, breaks etc.
Excemptions to this are, journeys of up to 30 mile radius from where you work. The weight then can be to 7.5 tonne MGW.
However you can only do this if you are not employed to solely drive vehicles. IE a Builder can so that he can get goods to his job, but a profesional driver would need a tacho fitted to do the same.
Vehicle delivery trucks are excempt from the above however. Though recovery trucks are restricted to 60 miles. IE a AA transit towing your motor home. It has to go on a loader for further travel.
It should be noted that the 30 miles radius from work is 'as the crow flies' so therefore you may well be able to do more miles to a job than 30 miles and still be within the law as few roads are direct and straight.
Theres alot more to it than this, but above is the basics.