As of the 1st January 1997, anyone passing their driving test that needs to tow over 750kg may need to take a further trailer test (B + E test). Those who do not hold this addition to their licence can face up to 6 points, £1000 fine and discretionary disqualification due to driving otherwise than in accordance with a licence. Further to this, because they are not licensed to drive the vehicle they are uninsured which means the driver can also face up to 8 points and a £5000 fine, plus discretionary disqualification. 
Failure to have an adequate licence is likely to invalidate insurance for loads carried; this includes horse healthcare, repair and replacement of valuable haulage and caravan contents.
If a driver is permitted to drive a vehicle he is not licensed for, then health and safety issues arise for which companies can be prosecuted for; which in the worst case scenario could be corporate man slaughter.

Driving licence rules and what you can tow
The rules on what you can tow are different depending on when you passed your driving test.
Licences issued from 19 January 2013
If you passed your car driving test (category B) from 19 January 2013:

MAM is the limit on how much the vehicle can weigh when it's loaded.
Licences issued from 1 January 1997
If you passed your driving test between 1 January 1997 and 18 January 2013, you can:

To tow anything heavier you will need to take a car and trailer test.
Licences issued before 1 January 1997
These should have the car and trailer entitlement.
Instructor Information:

Stephen Ramsden

I have been an approved instructor for over 20 years and keep my self up to date with changes in regulations and methods of teaching that give my candidates a high success rate.

Courses and prices:

This for someone with little experience
Its 12 hours long with the test included and no VAT to add


Suited for someone who can tow but just needs to acquire the licence
Its 8 hours long with the test and there is no VAT to add

Both courses include the current DVSA test fee of £115


For those with no experience then it is advised some tuition prior to a 2day course is taken
All prices include use of my car and trailer.

N.B. When looking to do these courses make sure the training provider does not sell you short on training time. This is a common complaint I come across e.g.  2 day courses only ending up with about half the training offered on mine, similarly, on my 1 day course. Also, I would always advise to seek an instructor with DVSA qualifications, as they have a better understanding of tests.