There definitely are some holiday hotels in some of the coastal towns that come with carers, hoists, etc. If you found one quite near to you it might be doable depending on your MiLs levels of continence and mobility.
Even if she needed a wheelchair all the time, and has adult 'nappies', as long as you have her straps for a hoist, and you can fit her chair in the car, and you and your husband are willing to change her, more and more places in England have what are called "changing places" they are very large disabled toilets, with a Some services have them, and places like the Trafford center have one - which might make a nice day trip for your MiL if you live near there - other shopping-malls probably have them too.
I can tell you, that living behind 4 walls, day in and day out, while your mind is still active, is very boring and can be very depressing too. But you, and other family carers like you, also have my greatest sympathy, even if you don't have to do the messy care work, it can be, and probably is, both very stressful and frustrating ‐ trying to arrange things like hospital appointments, and yes days out, and especially holidays away.
It can be done, but those hotels are probably very expensive (unless maybe your council would pay for some respite care in one?), and when I have seen photos of them inside they don't look exactly luxurious - that isn't to say that there aren't some very nice ones, I have very limited knowledge about them.
However if you are able to facilitate a little holiday for her that would be lovely, but don't make yourselves ill worrying about it if it is too difficult. If we are "lucky", we all get older, but we are not able to do the things we once loved doing. So if you can do them when you are fit and able, please do them x
ps. I meant to add that you should be able to find out about specialist holiday hotels on the Web, but maybe don't type in "specialist hotels", as goodness knows what may come up - no pun intended!