If you're after a cabrio, I'd go for something that was designed as such.
You could have a very nice Honda S2000 for much less than £20K - a wonderful car, proper handling... designed as a cabrio... but only 2 seats.
Or for not much more you could pick up, Jag XKR (probably a 4.2 but that's still 420hp)... it has 4 seats, not sure you'd want to use the rear ones though

£20K for a Golf ragtop, even a GTI or and R, makes little sense if you look around at what you can have for that sort of money... wouldn't be for me... I see their demographic as more Salon owner or Builder's wife
I can't imagine you'd be insisting on 4 seats in a cabrio... nobody will want to sit in the back with the roof down