From the official Transport for London website:=
The Tour of Britain is taking place on Sunday 11 September. Road
closures will be in place in central London from 03:00 until 23:00, in
particular in the Whitehall and Westminster areas, including Regent
Street, Trafalgar Square and Aldwych. Waterloo Bridge will be partly
Tube and rail will be the best way to get around central London on the
day, although stations close to the route will be busier than usual.
Buses in and around the event area will be diverted or will stop short
of their normal destination. A number of Santander Cycles docking
stations will also be out of service.
For full details, please visit tfl.gov.uk/tour-of-britain
If you like bicycle races this one could be fun.
If you don't like bicycle races, be very careful. If you try to run across the street where the race is you may be very surprised by jut how fast they go on the flat roads of London.