According to the Zwift forums, the new pack dynamics have now been turned on for the London map - whether it's a free ride, a group ride or a race (all previous tests were in specific events only). These are meant to make it more realistic so riders don't pass straight through each other, but at the same time changes it so no-one is weaving all over the road when overtaking. Also it is meant to make sticky draft a lot less prevalent, which I know would be a major boon to a number of people on here.
London is one of the selected worlds today and then again on Tuesday, Wednesday and Thursday next week; plus as I say, it'll also be in effect for events and meetups on that course as well, so might be worth a test and report back with if it's any good.