good news that you got the boat and got out.
I chcked the plate on the transom of my Honwave 3.8 and it says 20hp max.
Maybe they uprated the transom on this years model?
I felt the 15hp was a bit flat so thats why I went for the Yam 20.
It is a heavy engine though and am no9t sure I would want to go any larger.
I think the 25 and the 20 Yam 2 strokes are the same weight....but I had concerns about invalidating my insurance by exceeding the makers specification for outboard. You should bear that in mind. Bristol Docks are I imagine a flat piece of water.
My advice..take the boat out into the Bristol Channel in a bit of chop. I doubt that you would use WOT at all.
If you get time post a few pics....