The overall package exceeding 2 tonnes is not uncommon. from memeory the BMT package for a 625 was given a weight of approx 1980kg. 20 kg under the "limit". You will exceeed this limit with just fuel. It is the dealers obligation to supply you with a trailer that is legally capable of carrying the boat that they have quoted you a price on. If you allow for the weight that from the boat and trailer is placed onto the car via the towball, this is generally in the 120-150kg range is lowers the BMT weight. That is what a lot of dealers are allowing for in their calculations. That the specified weight of the BMT is going to be less due to the weight transferred onto the car.
Given the dealer has given you a written quote for supply of that boat and trailer it may pay to seek some legal advice about who is going to pay for the brakes.
Whilst the extra money is not pleasant the braking system will be much safer and of a great benefit.
Good luck
Russ