First of all, the good news - emergency medical treatment is free in the UK (for everyone including visitors). You can find more information here:
http://www.avert.org/freenhs.htm
However, this would not cover dental treatment - which we have to pay for as well (unless we are very poor).
Hotel rooms with showers (as opposed to bath tubs) seem to be the exception rather than the rule here, so your father would probably have to make a point of booking a "disabled" room each time.
I know of one (fairly well-known) national company that organises trips for senior citizens -
www.saga.co.uk. There may well be others.