To answer HP's original question "does anyone know anything about this practice" I'd say that like most "good lies" it has some truth about it.
I guess a broker can pick and choose his own "terms and conditions" but to attribute them to the BMF, RYA, MCA, etc is goiung a bit far.
It is normal with a "brokerage boat" that a viewing is conducted (running the engine and getting a computer readout (cost £49) is normal at this time) and if the prospective purchaser wants to buy the boat they sunbmit and offer.
If the offer is accepted then contracts are sent out and (normally) a 10% deposit is paid (this should be held in the brokers "client account" until completeion) and a date set for the sea trial.
Once the contracts are signed and the deposit paid, the boat has in effect been purchased. The sea trial is to check that the boat is as described (everything works) and is sea worthy. Anything that is not working will have to be fixed (at the vendors expense) or an adjustment made to the purchase price. Worst case scenario, if the boat is a total "duff" the purchaser can withdraw from the deal and get their deposit back.
Assuming everything is OK, the balance is paid and the boat can be delivered to the new owner.
There are quite a few important "bits of paper" in this process (contracts, listing forms, bill of sale, etc) and hopefully using a reputable broker will ensure you get what you pay for and it's all done legally.
However, although there is a fairly well worn brokerage sale path, we always try and remeber that the easier you make it for someone to buy the boat - the more likely they are to buy it! - maybe the broker HP visited needs a little reminder of that?