Can't believe so many posts without mention of the following: (all Autotrader sourced under £6k)
Saab 9-5 HOT Aero (250 bhp I think?) Leather & loaded, but perhaps not the machine to track too much...Bloody nice motor IMO...
SEAT Leon Cupra (180bhp standard?) Generic VAG 1.8t turbo, but old golf underpinnings, (not much room in the back?) mpg quite aceptable.
Volvo S40 T4 (200bhp I think?) Dull image but quite the sleeper, again possibly not the best track car but plenty of shove.
Honda Accord Type R (2.2 Vtec 200 odd bhp) Probably the best on track of this list, plenty of space -watch for the grey imports?
Octavia VRs - Same engine and bhp as Leon Cupra, well regarded, Kent Police lov 'em. Oodles of space.
Alfa 166 2.5/3.0v6 (190/220 bhp I think respectively?) Bella Alfa...lovely engine note, effin' cool, plenty of space. Is to Rovers what Monica Belluci is to Clare Balding. But many Alfa dealers
will sodomise you when its time for a service. Track days? 'eez fora angry yang mens weethout style'
Admittedly not all these would be pointy and steerable on the throttle at the track (all fwd) but blimey you can get some machinery for <£6k these days!