Firstly are you sure your over ? You have a max limit of 3,500 kgs all up, TrailerGuy has some excellent stickies on the laws - worth reading up on
Most instructors won't let you go right onto a test for two reasons
1) they will want to make sure your competent before using their equipment
2) they will need to make something out of it other than hire of equipment for 2 hours so usually insist on an evaluation before at least a 5 hr course regardless. The cost should be closer to £400. I can recommend one dependant on location.
You can book directly using your own towing equipment but the requirements are so high it's not worth it. Things like providing additional mirrors for the examiner, having the correct sized box trailer.
The test itself is not overly difficult with only one reversing manoeuvre (S type reverse/park) and a short drive. They also want you to demonstrate how to couple/uncouple the trailer the correct way.
Videos are available on YouTube which give a good insight into what's needed. Worth a watch.
Peter ~ Boatsandoutboards4sale ~ firstname.lastname@example.org
~ 07930 421007