The Highway Code's Rule 243 states that you should 'not stop or park where the kerb has been lowered to help wheelchair users and powered mobility vehicles, or where it would obstruct cyclists except when forced to do so by stationary traffic.'

Parking a vehicle fully or partially across a dropped kerb is classed as an obstruction and either the police or local council can enforce the contravention. If the problem persists, the council can mark a white line below the dropped kerb. However, the lack of line on the road or signage doesn't mean you can park there - it's just an extra measure the council sometimes utilise for problem parking areas.