Is there accommodation provided by the Hospital other than Foundation for over-night/long term ? Some are linked to Charities who provide flats close by. Maybe ring the Chemotherapy Dept on their behalf and ask. I don't think that these types of accommodation are age-led.
Is there a PALS-type group that liaise with families? Should be on the Royal Marsden web-site.
Dreadful story. Hopefully something *will* work. Again, a situation where the medical profession has dismissed a patient

They may find that small hotels will offer cheaper rates for several night stays. Look at the local tourist information web-site 'accommodation' lists.
Do they have a caravan? Is there a camp site where they could park up? Check the Caravan Club and others to find out. Some remain open 12 months long. Mobile home parks often have 'lets' either managed by the owners of the static vans or by the Park Management on behalf of the owners.