Public transit is only faster than cars when they have an "unfair" advantage. I'm guessing (I haven't been there) by London zone 1-2 you means trips where the subway (tube you call it?) gets transit out of the way. Other useful "unfair" advantage are trains - because they cannot stop - preempt traffic, and there are also transit only lanes where cars are not allowed. Anytime transit doesn't get some advantage over traffic it is slower because it has the same traffic plus the additional non-traffic stops it needs to make. For long distance trips transit is often faster.

The unfair advantage is not cost effective to grant to all cars, but still need to be acknowledged.