I agree about sustainability. The short term is one thing, but you need a solution which could work 3-5 years for both of them.
Is there any scope for them to "downsize" to sheltered or extra-sheltered accommodation people like this www.retirementsecurity.co.uk.
Nice flat where you can shut your own door, but lots of support in the background, eg 24 hour warden, restaurant, housekeeping, handyman, reception and other residents to provide company and support for your dad.
I simply asked everyone, GP, priest, everyone I came in contact with, for their ideas. Even the British Gas engineer. Community health staff will have visited most places and know what local options there are. Perhaps easier for me because my mum was on her own so there was really no question of her coping with a discharge home, in the way your dad might be able to "cope" with your mum.
The crisis then became quite a big project, but one that had a sort of satisfying conclusion. If there were an option which allowed on-site support for your dad, this enables him to provide better care for your mum and a better quality of life for him. For me it means there is a team who know my mother well and who I can phone or email. One day they will tell me she needs to move to a home but this now is put off until it is really inevitable.
It would also mean that should your mum need to move to a 24 hour care setting, your dad is not on his own.
Service costs are quite high, though much much cheaper than a care home. Attendance allowance and carers allowance are available, and though they may be reluctant to claim, if the money will make an important difference, they should claim.
And look after yourself....