This depends on when you passed your practical car driving test. If you passed your car test on or before 31st December 1996, you already have the B+E (car & trailer) category on your driving licence and can tow large trailers / caravans. Don't forget to stay within the limits of your car's towing capacity.
If you passed your car test on or after 1st January 1997, you can drive vehicles up to 3,500kgs MAM (with up to 8 passenger seats) with a trailer up to 750kgs OR tow trailers heavier than 750kgs as long as the total plated MAM figures of the vehicle and trailer added together isn’t more than 3,500kgs. The vehicle's own towing limits must also be adhered to (under Construction & Use Regulations).
There is currently a proposal to change legislation to allow all car drivers to have BE added to their licence without taking an additional test. This legislation change HAS NOT happened yet and you must only tow to the limit on your current licence to stay legal until then. The DVSA ceased conducting B+E driving tests from 20th September 2021.
If you passed your B+E trailer test before 19th January 2013, there is no specified weight limit of the trailer you can tow on your licence but you must still adhere to your vehicle's own towing capacity limits (Construction & Use Regulations apply here). The wording "the weight of the trailer must not exceed the unladen weight of the vehicle" was also removed around January 2013 as it had no legal merit but was, and still is, good practice to follow.
If you passed your B+E trailer test on or after 19th January 2013, The B+E category driving licence limits are a trailer with a maximum plated MAM weight of 3500kgs and an overall maximum gross train weight (car & trailer combined) limit of 7000kgs. In reality, these limits won't affect most people as even the best in class towing vehicles (such as a Range Rover) have a towing capacity limit of 3500kgs.
Using your own vehicle can make the courses more affordable for you. A lot of clients also feel more comfortable in their own vehicle rather than the extra stress of driving an unfamiliar one. Business clients often prefer their employees trained in their own company vehicles too. If required, a fully comprehensively insured (specifically for trailer tuition and tests) company training vehicle can be supplied upon request (subject to availability) at an additional cost. If you do use your own vehicle, it must meet the requirements for training and test. This includes appropriate insurance cover for towing a trailer under tuition and taking the DVSA test where applicable.