The Boxster is the obvious choice

but with that budget I would go for a higher mileage Audi R8.
Personally I would rule out the Elise as the 4 cylinder engines don't sound that great and you mentioned you won't be tracking the car.
I know you've said that the Viper is out and you may exclude other American cars, but how about considering a Corvette? Your budget would run to a very nice C6, probably the 6.2 litre LS3, this shape (but your budget would get you a later/low mileage/better equipped example): http://www.pistonheads.com/classifieds/used-cars/chevrolet/corvette/chevrolet-corvette-c6/6432293
A Z06 is within your price range but the roof is fixed and do you need that much power on the roads?!
+
That V8 noise :sweetheart:
Lightweight: alloy engine, fibreglass body
430bhp as standard with plenty more available with cams and headers
Decent handling (composite monospring rear)
Strong mechanicals, easy to service
Economy appears similar to our Z4M
Roof panel comes out for open air motoring
Value for money, turns heads, decent resale value, rarity in the UK
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LHD only
Interior design and quality are pretty lousy
Loads of choice but mostly in the US of course.
Maybe getting a bit too close to a kit car, how about an Ultima? http://www.pistonheads.com/classifieds/used-cars/ultima/all-models/high-spec-can-am/6544343
Lastly, TVR Tuscan? http://www.pistonheads.com/classifieds/used-cars/tvr/tuscan-speed-6/tvr-tuscan-mk-2/5420432