Classy mini-serial by Stephen Poliakoff, brilliantly done. Equally classy are the buses.
Episode 1:
Briefly seen from behind and inside at night is John Woodhams JT8077, a 1937 Duple bodied Bedford WTB:
Episode 2:
Buckland Omnibus TE7870, a 1929 Dennis ES with Brush body:
Buckland Coaches website notes that filming took place in Croydon and Whitehall, and includes some photographs.
Episode 3:
I'd welcome identification of this bus seen in the background of a sequence in episode 3: