More cities are declaring clean air zones excluding or charging more polluting vehicles if they enter the zone. London, Bath, Bristo, Portsmouth and Birmingham all have CAZs or will have shortly, and the rules vary from one to the other.
So, if you are planning to drive to a city check whether it has a CAZ and whether your vehicle is allowed, and if it is whether or not there is a charge.
https://www.gov.uk/clean-air-zones
Somerset Air Quality 