It's not that Londoners are more polite to foreign visitors than the inhabitants of major cities famous for their impatience like New York or Paris, it's just that we show our irritation in a particularly snotty English way. Tourists to the capital might even mistake it for politeness.
Take the fleeting one-sided exchange I had with a fellow-shopper in my local supermarket yesterday.
It's mid-way through a re-fit and temporarily the aisles have been narrowed. As I passed a man in his thirties, he muttered "Welcome."
Instantly I recognised the scenario. I had offended him and he was being sarcastic. He had allowed me to proceed, perhaps in deference to my age, at a point where only one person at a time could pass
I had been blind to what he considered a mark of good manners. He had expected me to say "Thank you."
(And so I would have done if my attention hadn't been elsewhere). He would have responded with "You're welcome." Instead by blanking him, he had treated me, once we had passed, with the heavily ironic contraction "Welcome."
How do I know this? I've been guilty of the same buttoned-up anger usually when going unrecognised for holding a door open.
Tomorrow the meaning of life.