Let me get this straight ... DH's friend tells DH to get his dad to move into your house. DH tells you he wants to install his dad into your house. You are not consulted or considered.
FiL has not even been consulted about it yet either, & is likely not to be allowed into the UK under the current harsh migration climate. Even if he is, his healthcare will not be covered. If he wishes to risk that, he will be sharing a room with his son, & spending every weekday alone in a house in a cold foreign country where he knows nobody else at all. His DiL will have no bedroom, his GC's will have to share their beds with their mother.
OR he could enjoy an extended visit from his son, retain the comfort of his own familiar home & surroundings, keep his maid & domestic arrangements, continue to enjoy the friends & social life he already has in place, & presumably, as the owner of a few houses, have the means to pay for any healthcare needs that may arise.
FiL would be mad to accept an offer to bunk up with his son in a hostile country with comparatively rotten weather & no friends.
Tell this to DH.
If DH persists, remind him that you live in Britain, where a wife is legally equal to a husband. That you do not wish to compromise the kids exams, your own training & exams, & that sharing rooms will certainly disrupt not just study but peaceful enjoyment of home life for all of you. That FiL will be cramped, cold, lonely, have no healthcare service, no maid (because YOU will not become the replacement maid) & entirely probably no legal permission to stay in the UK.
Then you broach this rotten old egg:
Nope, DH won't be doing anything...I speak from past and current experience. He never does anything. It's all me
Tell him you have had enough.
Tell him whether FiL can come over or not - that his assumption that he clicks his fingers on a whim & you do all the work has opened your eyes to how your marriage works & you are no longer prepared to put up with it.
That if he wishes to live with his FiL, he can either go to India & move in with him, or he can take on the work, admin, risk & bother of trying to sort out visas, healthcare, sleeping arrangements, shopping, cooking & cleaning.
Or that he & FiL can buy their own UK house or flat & look after each other, as you want a divorce & the kids are staying with you in the house.
You don't have to accept this OP. Use the suggestion as an opportunity to review the dynamics of your marriage. If you don't, FiL or not, when you are older & the children have left home, you are likely to end up as your DH's full-time maid & carer. How does that prospect feel?
I am so sorry you are being browbeaten like this - I'm also aghast that your FiL may be about to be encouraged to leave a comfortable life full of familiar friends & routines, for a lonely existence in a cold country where he knows nobody & has no security of tenure or health provision.