Dilemma ! you don't say if you have discusses this with your parents, and what they would, if possible, like. It may be, as suggested here, they could sell their house in order to pay for an extension to yours ...if that is what they would like ..you would then have the granny annexe for your in laws if that is necessary ...and it could add value to your house if and when you decide to sell (and I think in the UK granny annexes are at either no, or reduced, council tax ?) ......We chose to move to France, and in any case, my daughter has not spoken to me for 10 years, my husband's daughter makes it plain she has her own life ...there is no way either would wish to care for us (and to be honest, we would not want them to !) ...as in the UK, any form of care is expensive ...but there are schemes whereby people 'rent' a room to an elderly person .... and schemes whereby an elderly person takes perhaps a student in rent free, so they can help with things like gardening, dog walking, driving, etc .....In my case, when I lived in the UK I offered my dad, who was by then a widower, a bedroom, small lounge and en suite bathroom in my then large house ....he was 75 at the time. He declined, saying he would rather live on his own, so he sold his house and moved into a retirement flat with an alarm system. He was really happy there, having other people of his generation to socialise with. I was therefore pleased for him ...he would have been isolated I think, if he had moved in with me as he would have had no friends locally, so for him it was the right decision.
It is difficult now, especially for the sandwich generation, if you have children of your own, or a full time job then it is really not possible to uproot your family and husband ...maybe you could investigate, with Age Concern or whatever it is called now, and Social Services, private agencies, etc what help might be available where they live and what would be the likely cost ...and if any help, if they need it, towards costs, might be .....if difficult where expense is concerned, are they claiming everything to which they might be entitled ?
I would not be too quick to move away if your husband wants to stay ....discuss with him what he would do if it were his parents .....but you may find you end up on your own and regretting this decision. Do you have friends with elderly parents you can discuss this with ? Everyone is different, I know, and good luck with your decision ...weight it up carefully, which I am sure you will do !