Was just looking at the old buses again. There were 2 heritage routes in London that ran the original Routemasters, route 9 and route 15. 9 was discontinued in July 2014. 15 in 2019
https://en.wikipedia.org/wiki/London_Buses_route_9_(Heritage)
https://en.wikipedia.org/wiki/London_Buses_route_15_(Heritage)
Here is a vid of a Route 9 journey - https://www.youtube.com/watch?v=H3T6Xs2r19c - also see 2nd pic. The bus goes to Picadilly and then turns right to go south along Haymarket to Trafalgar Sq. See pic 3. BUT if there was some reason Haymarket was closed, Oxden St is a one way which would force the bus to carry on along Coventry Street and then turn right onto Whitcomb St to head to Trafalgar Sq.
This could explain the old bus in the Brennan lookalike/GH pic.