I think it's tough to do a big event on closed roads and have it be an interesting route too. If it was all narrow twisty lanes it's difficult to keep it safe and deal with any incidents. Having the start in central London is pretty constraining too.
I think there are more interesting options North West of London - some of Hertfordshire or the Chilterns
Some sort of route rotation every year would be a good option to keep things interesting, rotating a set 3-4 routes around London. Would keep the country drivers happier as well if they only had to tolerate people enjoying themselves on their patch once every 4 years.
I think it's tough to do a big event on closed roads and have it be an interesting route too. If it was all narrow twisty lanes it's difficult to keep it safe and deal with any incidents. Having the start in central London is pretty constraining too.
I think there are more interesting options North West of London - some of Hertfordshire or the Chilterns