This doubledecker seen in an episode set in 1938 just before the outbreak of World War II is ostensibly a London Transport bus, but the livery is wrong and the registration (LYL642) is fake. It does sport an AL garage plate (Merton): Episode 2.02: The Love That Pays The Price
The bus seen in episode 1 is now revealed as CCX777, a 1945 Daimler CWA6 with Duple utility bodywork new as Huddersfield Corporation 217. Although this type did serve in London during the war, the D class did not begin to enter service until 1944. CCX777 carries fleet number D130, for which it has on occasion carried what would be the correct registration HGC257, was at the time of filming with Quantock Motor Services. The real D130 (HGC257) entered service from Merton (AL) garage in 1945, so although this appearance is an anachronism, it is more correct than at first appears.
(thanks John Wakefield)