heres the basic rules according to all info i can find:-

all trailers need suspension (this included tow dollys etc and cars become a trialer when used with a A frame)

all trailers capable of carrying over 750kg must be braked so if someone says its will carry 2 ton but it has no brakes it cant it.

A frames and dollys are illegal for transporting vehicles BUT are perfectly legal when recovering a broken down vehicle unless they are braked (dollys) or have the brake appling attachment ( a frames )

any one who passed there test after jan 97 can only tow a combination of the tow car and trailer gross weights up to 3500kg and the gross weight of trailer must be less than the kerb weight of tow car but you can drive a 3500kg vehicle and tow up to 750kg as well

these are only the basics and i would suggest a quick google search and you will find all the info you need