1—All cities are multicultural, they always have been. Its their nature.
2-So?. Since when was London the whole UK anymore than New York is the whole US?.
I'd suggest that a larger city would have an easier time with logistics as the relative impact of a large influx of peeps on the existing infrastructure would be smaller.
London has a larger airport (and maybe a 2nd one), more lodging and more public transportation on an absolute basis.
The security issue could easily be the opposite for nearly the same reasons (excepting the MUCH larger number of militant islamists in London which makes it even more difficult to control/prevent "events" there, even if the security team weren't recently beefed up at the last minute).