Can’t say I have a well thought out answer but I’d say that the Red Wall was won because Johnson represented an aggressive Brexit. The Home Counties have remained largely unchanged in the meantime and was pretty much 50/50 regarding Brexit.
For Labour meanwhile London was 60/40 remain and as mentioned the Red Wall was hard Brexit and no longer holds liberal values, I think the change there is probably going to last a generation.
Longer term I actually think the Lib Dem’s will start to chip away at the Home Counties as the Conservatives **** business agenda starts to take a grip. Part of the reason Labour should work with the Lib Dem’s, they will break through in parts of the South that Labour won’t.