Looking over the options it appears I have several:

Hobie 13 (A Europe-only model?)
This appears feasible though I'd have to find out how easy it is in practice.

Hobie Wave classic
I think this boat is a little too heavy to put on the roof. My rack can only officially hold 80Kg (approx: 180lbs)

Hobie Bravo
This seems a little small for taking a friend out or preferably two kids. Besides I'd like a trapeze.

Hobie Adventure Island
This boat is like a small trimaran. It doesn't look like what I want. More of a regular beach cat.

Topcat K3
Now this boat really appeals to me. It appears extremely practical and the Texel rating is not too bad either.

Windrush 14
I don't think they ever show up in this neck of the woods. Otherwise it looks great.

KittyCat
Too "classic" for me. Makes the original Hobie look hi-tech.

Sprint or Dart 15
Very interesting boats. I'll keep my eyes peeled for a second hand one, probably a Dart then.

Someone recommended an A-cat via PM. I like that idea but could only afford one if I built it myself. Even then it would be stretch (in many ways probably).

I also found a Nacra 4.5 on the 'net. Anyone know if this is car-toppable? Size and weightwise it could be...