I forgot .... don't forget about the larger Colvic Countess's .... very good accommodation and sailing is not racing standard, but they hold their own. Another benefit is they don't suffer keel problems like the Westerly's .....price is slightly lower as well.
as always, the readies. For £18k I think you're going to struggle to gain much in real terms (ie perfomance and accomodation), over your Centaur.
I agree that the Hunter 27 is a really good boat, but is there really that much of a gain?
The Southerlies will be way over budget.
The Countesses are, how shall I put this diplomatically, ugly floating caravans (to my eyes, let me stress). Many were home finished to a basic standard, and you won't get a 33 for £18k.
To me there's one obvious candidate, but you've discounted it already. The Iroquois catamaran is a very capable sea boat, has performance that makes Sigmas look sedate, and has amazing accomodation.