Audrey Collins, the (UK) National Archives family historian always worth paying attention to, has written an instructive blog post Missing from the census? on TNA website. Part of the advice:
"You can get a rough idea of which parts of a census are missing by searching in Discovery, our online catalogue, using the keywords ‘missing’ or ‘wanting’, and restricting your search to the reference for that census; HO 107 for 1841 and 1851, RG 9 for 1861, RG 10 for 1871 and so on, up to RG 14 for 1911. This will usually show where all or part of a parish or district has not survived, and you will see that 1861 has more damaged or missing parts than any other year. The missing parts even of this census still only represent a tiny proportion of the whole census, but that still amounts to an awful lot of people."