Simon,
A difficult problem!
It depends a bit on what vehicle is doing the towing - each one usually has a recommended trailer maximum nose weight. An elderly VW Golf is only 50kg but a more modern Golf is 75kg and a VW Touareg is 140kg. It also depends on how far you will be travelling. If you think of the structural stresses that having a 25kg outboard bouncing around on the back places on the BRe's transom, especially on potholed UK roads, it's best avoided if at all possible. If the nose weight is already 50-75kg and you remove the outboard, unless you're driving something mighty, you may be over the recommended maximum.
The optimum solution is to find a trailer centre (Wessex Trailers?) that can do a quick job of moving the wheels, brake cable etc further forward so that the nose weight comes within acceptable limits without the outboard engine on the back. Failing that and as a short term and short distance solution, no outboard on the transom but full water carriers tied down at the back?
The Caravan Club recommend a nose weight of 5-7% of the trailer’s laden weight. So if your braked trailer loaded up with your BRe and gear is, say, 850kg, that suggests a nose weight of 42-60kg. Unbraked trailers are easy to adjust for nose weight but braked ones are a bit more complicated.