I was thinking about your conundrum today whilst driving my commercial TD5 Defender around town.
These are great for working offroad and towing but as an everyday car they are awful - noisy, juicy, heavy great lumps.
It struck me that 4x4s are pretty much a prerequisite for towing up slippy or less than great slipways but less so for straight towing. As you will only be towing a couple of times per year you could save your money on 4x4 ownership (cost and diesel) and simply get the boat lifted out by a boat yard or towed up the slip by a friendly local 4x4 owning forumite. Many boat yards/clubs have tractors to do the latter.
My second thought, after filling up, was consider the running costs the rest of the year before you buy yourself a towing car - my TD5 only does about 30mpg and the annual fuel bill would be crippling if I didn't pass on the cost - if you run an economical car you may end up still saving money in comparison even if you hire a van/car for towing a couple of occasions.
You certainly got a discussion going here didn't you? I sort of zoned out of the technical/legal stuff, but then I am a bit dense.
PS you have got me looking at alternative vehicles now. She's going to kill me.
Fifteen men on a dead man's chest
Yo ho ho and a bottle of rum