This is from the Practical caravan magazine, see the bit at the end saying that the 750kg rule is the one which gives the greatest weight capacity in practice:
DRIVING LICENCE REQUIREMENTS
This is a complex and much-misunderstood issue that has often been incorrectly and misleadingly stated by the general press; even government departments have been known to get it wrong.
There are currently two different sets of rules that govern what you can drive on a Category B car licence without taking an additional test:
Licence gained before 1 January 1997
If you gained your driving licence before 1 January 1997, you will be entitled to drive any vehicle-and- trailer combination up to 8250kg total weight, of which the towing vehicle’s maximum weight must not exceed 7500kg. This right remains while you still have this particular licence, but is removed if it is withdrawn. This means that if you are disqualified or made to re-take the test you will lose the right to drive heavier vehicles without further testing and will revert to the post-1997 rules (see right). For pre-1997 licences that are still current at 70 years of age, there are some extra medical rules if you wish to renew an existing right to drive vehicle and trailer combinations of up to 8250kg maximum weight. To keep this entitlement you must take the D4 medical test, which your doctor will charge you for, and you must be able to pass the existing standard car eyesight test. Unless you satisfy these conditions you will also have to revert to the post-1997 rules. Details can be found on the DVLA website (
www.dvla.gov.uk). Look for form INF40, also available from post offices or by telephone (0870 240 0009).
Licence gained after 1 January 1997
If you gained your licence on or after 1 January 1997, then you will be restricted to driving or towing with a car (or light commercial vehicle) with a maximum weight of 3500kg. Note this is not the actual weight as loaded, but the vehicle’s maximum allowable weight, or gross vehicle weight, as set by the manufacturers – you will find this in your car’s handbook.
You can tow a trailer or caravan with this licence, but its maximum weight must not exceed 750kg – giving a total possible maximum outfit weight of 4250kg. If you wish to pull a caravan weighing more than 750kg (in practice, this covers all conventional caravans), you must follow the restriction that the maximum allowed weight of the car-plus-caravan combination weighs no more than 3500kg, and that the caravan’s maximum weight does not exceed 100 percent of the car’s unladen weight (this is not the same as the car’s kerbweight – have a look at the panel on this page for a full explanation).
What makes this rule appear rather odd is that if you wish to tow a trailer that weighs over 750kg, you actually end up with a lower total limit for the car-plus-caravan outfit than you would if you were towing a trailer under 750kg, where the total can be 4250kg.