I know the Thames can look lovely, but below Teddington its a rough place to be. The trip boats kick up some serious wake and combined with the water running at over 5kts, you are going to need a decent sib to enjoy it.
Realistically I would be looking at 3.6m minimum with at least a 15hp engine, although there is a speed limit you are going to need to be able to plane just to make headway against big tides.
Everone has favourites but if you want to fold and carry in the back of a Ka (with seats folded) I would predictably suggest a Aerotec 380, probably the best transportable SIb you can get, combined with a 15hp 2 stroke, it will do the job and just about fit in your Ka.
If you want to tow (not even sure the Ka is type approved ?), go for something bigger, Futra Mk2 or Mk3, Honwave T40 etc.
Lots of options used and can be really good value this time of year. When you find something, ask for specific advice on here, it will help you avoid the usual pitfalls.
I havent already read the 'Which SIB' sticky.