Electoral registers, as well as directories, are a good source to track people over the years.
Ancestry (which you seem to have a sub for) for have them for London as well as many others.
from 1918 there were 2 a year Spring and Autumn
qualifying date to be included was 15 January for the Spring register which came into force on the 15 April
15 July was the qualifying date for the Autumn register which came into force on 15 October
from 1928 the dates changed slightly
1 Dec and 1 May for the Spring register
1 June and 15 October for the Autumn register
If you want to see 'which' register a likely entry was in, use the filmstrip beneath the main image and scroll back till you see the header page which will tell you
Boo