Brian James didn't refuse £2.2k from a friend when he bought his- hire companies will usually ask for proof if train weight is over 3.5ton though.
A good friend completed his trailer test, cost almost £1k all in. If you're post 1997 you must meet the following rules:
Car & towbar type approved for towing 'x'kg
Car able to tow > MAM of trailer
Car + Occupant + MAM of trailer < 3500kg
Basically with a Polo weighing in at 830kg max, you're easily allowed to tow with a 1500kg MAM trailer, using a normal family size saloon (Volvo S60, Audi A4 etc).
In my experience, providing the trailer, the tow car, and what's on the back of the trailer look reasonably good, everything is strapped down, lights working, and you're not being a bell end doing 90mph the police will be fine- I'd say 90% aren't aware of the towing weights/laws.