Get something with a turbo which can be easily *ahem* adjusted back and forth through the power range when you need/want to then switched back before an MOT. I'd say an Astra H Coupe 1.9/1.7 CDTi. Low tax, Low insurance, decent looks and quite decent performance while retaining fantastic economy. The bonus of the good diesel is that you can step on it and get performance or drive like a minicabber and get amazing MPG. I'd also suggest any french diesel over a German one, the only problem is I know most people have strange feelings towards french cars for some reason unknown to me