I suspect you're right that Britains do not hold many of the boats themselves but there are still plenty enough floating around (no pun intended) with the retailers that ordered them in.
As with the AZW limber piece coming out soon, Richard quoted figures of 70% already pre-ordered. This will however include orders made by retailers, so 30% doesn't equate to the actual stock that is still or will be available.
The Nile Boat, as lovely as it is, is a huge investment as far as our hobby goes. When you think about it, that's the equivalent of four, maybe five, K&C tanks/vehicles that sell so consistently profitably in the secondary market. I believe some retailers will need to take a hit in order to shift them especially if they intend to order in the next huge investment i.e. the gun limber.
Actually when I think about it, I passed on the Nile Boat and instead have nearly bought the entire Australian Lighthorse range with the money. That's incredible really.