We'd recommend a Mazda MX-5. It's the UK's favourite sports car for a reason: it's great to drive, represents excellent value for money and is exceptionally reliable. From experience, a Mazda MX-5 should manage three or four weeks without being driven. Your budget will get an early example of the latest (fourth-generation) model which still looks and feels like a very modern car. Don't dismiss the older third-generation MX-5, though, it's a bit more spacious and more comfortable than the later car. If you'd prefer something with a premium badge, take a look at the BMW Z4.