Some of these suggestions are a bit unrealistic;
350Z's - the only ones under about ?17k are Jap imports (may not be a problem to you though)
Mk5 GTI- for ?15k!!??? Not a chance.
GT170 - ditto. There are none of these on the used market. Drive the deal will do you one for a little over ?17k new though.
A3 3.2 S-line- again no chance for ?15k.. There is one listed on the trader for ?15750 but it's not an S-line, just a sport. S-lines start at about ?18k. Dealers will not take ?1500 off the price of a used car (like Andy suggested), especially not something desirable like an A3. Having said that, an early 3.2 Sport is achievable for ?15k.
I'd have a VX220 if I had 15k and didn't need practicality, but Focus RS and Leon R both sound like good choices if you want the option of the back seats (think I'd lean towards the Leon though).
Someone suggested a Megane 225- you can get these new for not much more than ?15k and they are great fun. Not the best interior but good to drive.
For that budget though you've got too much choice![
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