To be fair I wouldn't call Westminster a normal residential area where people normally live its for the vast part offices,government buildings, shops so why would a homeless person find themselves sleeping rough in that part of London maybe it's because of all the tourists which mill around that area. Keeping city centres clear of people who choose to target tourists is pretty much what most countries in Europe do. Maybe you've not been to London recently and seen the aggressive begging which goes on, drug addicts and pickpockets that inhabit that area.