I've found a few prisoners and asylum inmates on censuses. I think its only in 1901 that I've found initials only, but found my chap as he has a small village as his birthplace.
I haven't got anyone in prison/asylum in 1911 so don't know about that census.
People with prisoner ancestors may be interested to know that the gaol registers are usually at the relevant county archives. These commonly give the prisoner's name, age, birthplace and a physical description which can be very detailed. Some archives, such as Bedfordshire and Gloucestershire, have searchable online indexes.
Additionally, The National Archives at Kew has quarterly returns for many prisons which would give the prisoner's name and a word or so about his/her health.