The Housing Care website has a decent directory of all retirement housing for rent in the UK, searchable by County and Local Authority area.
housingcare.org/elderly-uk-sheltered-housing
This directory includes independant living schemes (what used to be known as sheltered housing), and purpose built housing for older people without the communal lounge facilities etc provided by independant living schemes.
It includes retirement housing for rent provided by both social landlords (HAs) and private providers. The rents of the former will be more affordable than the latter. The ease/difficulty of getting a tenancy with the former will depend on local availability and demand. The process of applying for the former will often be via applying to the Local Council who will have nomination arrangements with social housing providers of rented retirement housing, though some social housing providers of this type of housing will accept direct applications (in my own experience, Anchor Housing does).
For the most part, only 1B or studio accommodation will be available. The studio accommodation can be a really good option if the person has, or is able to buy, size appropriate furniture. My lovely Dad had a Anchor Studio flat when he moved from the family home around 20 years ago. He loved it. Unfortuntely Anchor reviewed their stock in the area where Dad lived and decied to close this scheme, Dad was rehoused in another flat by another social landlord, This was a 1B flat which, in my view, was less spacious than the studio flat.
MOnica is right in flagging up that in some areas, social retirement housing schemes have been converted for general let purposes due the the decreasing demand from older people for this type of housing. By the same token, some social landlords with 'hard to let' independant living schemes originally designed for older people started letting vacancies to the younger cohort of older people i.e. the over 50's who had different needs from their later older age neighbours, often addiction and /or mental health related. This was the case with the second flat that Dad moved to - it was not a good mix, and is something to be aware of if you are considering applying for social housing rented. retirement housing.