In message , Helen Deborah Vecht
writes

I'm afraid forward planning might be the only way to go. If Aged P has
access to a computer and the Internet, having the maps needed in .pdf
(downloaded from the TfL website) form which can then be magnified &
displayed on the screen might help at home.

Of course that's no use when travelling though:-(

Anyone with eyesight (or any other sort of problem) can ask at the
station for help with travel advice and journey planning.
They can also ask for assistance at interchanges and final destination,
at their station of origin.
The request will be phoned to the station/s concerned. It's standard
practice.
--
Kat Me, Ambivalent? Well, yes and no.