It is not illegal to park on the pavement, except in London where laws have been passed to make it an offence anywhere where it is not specifically allowed. But it is not quite legal either.

Parking a vehicle partially or when wholly on the footpath by the side of the carriageway is legal unless it breaks other laws of the highway.

It’s fine unless “a vehicle or trailer is left in a dangerous position or a vehicle or trailer causes an unnecessary obstruction.”

It is illegal to park on the pavement where yellow lines prohibit it. 

Double yellow lines make parking anywhere on that stretch of the highway illegal, not just the vehicle carriageway, but the footpath as well

It is also illegal to drive on the pavement, and you might think, logically, in order to park on the pavement a driver has had to break the law by driving on it; but in effect, unless you do it right in front of a police officer, you're unlikely to be in trouble

But if your vehicle is in a dangerous position or is causing an 'unnecessary obstruction' you could be in for a fine

A local council-issued Penalty Charge Notice costs between £50 and £130.