If you want something similar to the Boxter then perhaps my suggestions from the VW Group stable won't interest you, but you could consider:
A new Polo GTI (1.8T 150BHP) 3/5dr
A new Polo 1.9 TDI (130BHP) 3/5dr (economy!)
A mk4 Golf R32 3/5dr
An Audi S3 (1.8T 210BHP or 225 BHP from 2002) 3dr
An Audi A3 3.2 quattro Sport (3dr, 2yrs old)
An Audi TT Coupe or Roadster (Previous model) not forgetting the entry level 150BHP front wheel drive version that was about �22k new.
A SEAT Leon 1.8T Cupra (1 yr old - 5dr, same engine as Audi S3)
A SEAT FR (Formula Racing trim for Ibiza, Leon and Altea models) Possibly not too many used models around yet.
I've missed out the VW Golf GTI though because �15k won't yet get you one from a franchised dealer. Not too long to wait now though, as this car's been on UK roads since Dec 2004.
No point mentioning Skoda as that would be too much of a come down! Good enough cars though.