I don't think it applies exclusively to the elderly, but AFAIK the rent-a-room scheme means you can rent a room for under £100 without declaring it for tax etc, if it's under £100 it's legal for it to be cash in hand, so if I'm right about that then she can charge any amount under £100 + whatever services she wants in exchange for lodgings without having to declare it anywhere
The Lady magazine is where positions like this are usually advertised
MIL lives on her own near clapham junction and has a spare room. She is mostly ok living on her own but she is feeling increasingly anxious and finds it hard to stay on top of things like cleaning, grocery shopping etc. She doesn't want to move out but agrees that a lodger would be a good idea for the company and as a sanity check.
I seem to remember hearing about a scheme whereby elderly people let out their spare room to lodgers who, in exchange for cheap rent, agree to spend a few evenings a week in and do things like make sure the fridge is stocked up. We will sort out a cleaner for MIL so there won't be huge amounts of housework.