There are 13 road tax bands in the UK, which are marked from A to M. The road tax band A is the cheapest band while M is the highest end. The band of your car depends on its fuel type and CO₂ emissions. The lower the CO₂ emissions, the less road tax you need to pay.
However, if a car is registered before 01/03/2001, there are only two types of road taxes according to the engine size: £130/year for engine less than 1549cc, £215 for engine bigger than 1549cc.
For more, check the DVLA site.