Use electoral registers
To fill in gaps between censuses and other sources, search for your ancestors in electoral registers online or at local record offices. This is useful for tracing eligible voters forward in time (remember that voting eligibility changed during the 20th century, so be aware of when changes happened). When a voter disappears from the register, but other family members remain at the address, this could indicate that the relative has died or moved away.
Findmypast hosts ‘England & Wales, Electoral Registers 1832–1932’, a growing collection of the names of approximately 220 million voters: bit.ly/fmp-electoral-registers. The site has also recently transcribed its electoral registers for 1920 covering the whole of the UK, and TheGenealogist has done the same for 1921. Ancestry also has a large collection including London, Liverpool and the Midlands, and has recently added Aberdeenshire and Cambridgeshire to its collection.
For tracing more recent relatives, Findmypast’s UK Electoral Registers 2002–2014 is essential: bit.ly/fmp-registers-2002-14. The index includes address, age, years of occupancy at that residence, and names of any other occupants.