Thanks for pointing that out lacb - I've asked for clarification.
A quick browse shows currently Islington have gone the furthest with a blanket 20 mph limit, including main roads, but not including the tfl managed roads.
Southwark are bringing in a similar scheme next year.
One small step at a time I guess. Then the pressure can be ramped up on tfl.
The difference between being hit by a car at 25 mph rather than 35 mph is very significant, as the impact energy increases with the speed squared.
So a car will do twice as much damage at 35 than 25 mph - and at these speeds this significantly determines who lives and dies - it is important.