It's a bit of a moving target. Looking at a medium/large size saloon. The cars that seem to make the most sense are diesels, e.g. BMW 320d, Honda Accord 2.2, Saab 9-3 1.9, and so on. Need to be careful with the exact year and model of the car as the CO2 varies. Also the UK bands A/B/C are different to ours.
Having said that, not a high mileage person so petrol will do. Second hand prices here seem to be dropping alot, so it may not be as worth while.