I am a professional genealogist and hope that I can help with census release dates.
1901 and 1911 were released as soon as the technology was in place and transcriptions had been done, and licenses obtained by the various genealogy sites.
In 1920 the Census Act placed a 100 year closure on all census returns, so that covers the 1921 census. My latest information is that it is probably going to be released into the public arena in Jan 2022.
The 1939 Register is a very valuable 20th century tool. It was taken on the outbreak of WWII on the 29th September 1939 to facilitate the management of the population during wartime conditions. Each person recorded was meant to give their exact date of birth - which census returns do not do, as they as 'how old are you?' Bear in mind the 1939 register, as any record, is only as good as the information supplied. My grandfather was notorious for adjusting his date of birth to suit his circumstances - ie which granny he was married to at the time. He was born in 1881 - I know that - but he is recorded as anywhere between 1879 and 1885 - so beware!
The 1939 register was updated right until the 1990's when digitised - it was also used to help assess the population for the NHS in 1948. When I say updated I mean that often females married names are written alongside their names at the time the register was taken. This register can be found on most genealogy websites.
If you do not want to pay for subscriptions, try looking at the free site 'familysearch.org' there is a lot of interesting info on there. and it also links to Find my Past where it is easy to pick up newspaper articles. Good Luck and Enjoy!