I did a fair bit of online research a few years back, at that time .gov had guidance for parking in England and Wales, and that didn't define if the whole vehicle or just the wheels needed to be inside a bay. On ni.gov there used to be a page which had photo's showing what was allowed and what not allowed, and this clearly showed it to be wheels, but NI's laws are not always the same as rest of UK.
I've read online posts of people getting tickets for overhanging a bay, but this might be a over enthusiastic warden, as I know (having asked) that some councils at least only go by the wheels.
Myself as a preference will overhang a grass verge, never a pavement, if I have to overhang another bay, or even park over two bays and I'm not sure of the rules for that area I will phone if a number is available on any signage, some don't allow two tickets.
BTW a word of warning if going to Netherlands, they are very strict on parking, if it says 'carpark' it means cars only, they even police free carparks and even if the entire vehicle is inside a bay, if it's the wrong class of vehicle you get nicked.