Pickthatup, if your FIl has funds to pay for his care privately theh in theory there is no reason, as far as i know working in this field, that he should not choose to pay you a wage to be that carer.
What I would be asking though is
Does he still have capacity to made decisions regarding his care needs and his preferences, ie be able to understand and retaing the information long enough to make a proper decision? If you are unsure then he should have a psychatrist who could help you with that. if he does not i would tread more carefully.
Secondly, do you or your husband have any duties or rewsponsibilities regarding FIL finances, do you help in either formally (EPA/LPA) or informally? Again if yes to either I would seek extra advice to confirm no conflict of interests (to protect yourself as much as FIL)
Bear in mind as well that if/when his savings drop below £22,250 he would be eligible for funding by Social services, they may or may not be willing to continue to pay you at the rates you have been used to, possibly not at all if they only use registered domiciliary care agencies, although you should be able to get round that by asking for Direct payments.
Good luck, I am all for people being cared for at home for as long as possible.