Vauxhall VX220? Looking on Auto Trader they tend to start at £16k though.

Toyota Supra MkIV? Goes like *stink* and IMO the best looking Jap car around. Available from £9k for the twin turbo (apparently the non-turbo is not worth bothering with).

I'll have to stick a word in for the Corvette, as for smiles-per-mile you can't buy better, even though it will treble your fuel bills overnight.

Gareth