Parking on the footway outside of London is not necessarily illegal. Action can only be taken if:
The vehicle is parked on the footway adjacent to a yellow line waiting restriction - this is because yellow line represent a waiting restriction from the centre of the road to the very back of the public highway, including the footway or any verges. If a vehicle is parked next to a double yellow line, enforcement can take place at any time. If a vehicle is parked next to a single yellow line, parking on the footway will be restricted during the same hours as the single yellow line.
The vehicle is parked in the Controlled Parking Zone - there is a requirement for all vehicles to park in a marked bay during the operational hours of the zone. Any vehicle parked on the footway during these hours will be liable to receive a Penalty Charge Notice.
The vehicle is causing an obstruction - the Police have retained their power to issue Fixed Penalty Notices to vehicles causing an obstruction. This includes on the footway. However, the Parking Service do not have the legal powers to deal with vehicles purely causing an obstruction and any enforcement will subsequently be at the discretion of the Police, who can be contacted on the non-emergency number of 101.