David Thrower notes an appearance by London Transport RTL1050 (LLU829).
Signoret's character catches Southdown 1182 (BUF82C), a 1965 Leyland Leopard with Plaxton body, to London's Victoria Coach Station:
When she arrives at Victoria, she disembarks and walks across in front of 1182 and a South Midland Bristol LS6G/Eastern Coachworks, then as the camera
pans across, a Maidstone and District AEC Reliance and another from Black and White Motorways:
She's then followed out onto a London Transport AEC Routemaster RML displaying route 88 on Buckingham Palace Road:
When she arrives in The Strand, she's seen getting off an AEC RT, although the interior footage is of a Routemaster:
Lastly the sighting of RTL1050 is from the rear, in the dark: