What's the reason for the building regulations? Safety? To protect the skyline? So what little sun London receives it can get to street level?
Quite a lot of buildings are protected for historic reasons, even ones that don't really look nice. Planning laws also tend to require maintaining the "character" of the area, so ripping up old streets/buildings and replacing them with shiny new ones is generally quite difficult. You see a lot of new or refurbished buildings that still maintain the original front of the old one as a façade.
There are also regulations that
protect the skyline; there are certain things that you must be able to see from certain places, and several of those things are right at the heart of the city. There is also a lot of political and social opposition to high-rise buildings, for purely aesthetic reasons.
Finally, England is full of NIMBYism, and our laws not only reflect but encourage that. Developers have to bend over backwards to cater to local residents when planning new buildings, and in a lot of London, those residents are not exactly keen on a massive great tower block being put next to them, blocking their views, causing traffic, and flooding schools. Local councils don't really benefit much from new buildings: residents are almost always "unprofitable" from a tax-and-spend POV (councils are partly funded by central government, for this reason), so their only incentive is to cater to existing residents, who already have a vote.
Honestly, I think it's just that London is relatively small compared to most of those other cities.
Though it might have something to do with the English having higher standards of living, and that they thus "won't accept" higher population density.
Yeah, and the two are surely related. Part of the reason that London hasn't grown a lot more are planning laws that protect green belt areas. London's contiguous built-up area would surely have expanded well beyond its current boundaries, if it weren't for the strict green belt area of protected land surrounding it. It's also true that most people, even in big cities, yearn for things like gardens and trees. So you have laws that keep London small in height and other laws that keep London small in area too.