Is 'a London bus' a possession? and if it is, why wouldn't it be 'a London's bus'?
thanks for you help
Welcome to the forum.
No, it's not the possessive. The bus doesn't belong to London. It operates in London.
I really like the New York subway.
The Madrid Metro is very efficient.
You could use the possessive in something like "London's buses will have to avoid Hyde Park Corner this week due to essential roadworks".
In the singular, though, you can only call it "a London bus".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.