The vehicle manufacturers specify a towing limit for the vehicle

In theory up to the maximum permitted in your handbook, which might be 200% of the unladen weight of your vehicle!

In practice however, it depends upon your experience and confidence.

There are two factors that restrict your towing weight.

The vehicle manufacturers specify a towing limit for the vehicle which you may not exceed (for example 1800kg).

The vehicle manufacturers also specify the maximum gross train weight of the vehicle again which may not be exceeded (for example 3600kg).

Therefore, using the examples above, so long as your car and caravan, in total, weighs less than 3600kg, of which, no more than 1800kg is the caravan then with a licence first issued before 1st January, 1997 or if you have passed a supplementary towing test, then you can legitimately drive the combination at 60mph on the motorway.

For someone whose licence was first awarded after 01/01/97 the regulations are different in that they are not allowed to tow a vehicle that weighs more than the kerbweight of their car without taking a further test.