if could get a £40/50K loan say if I could afford £200-£300 a month, but to save up that kind of money would take 20 odd years which is not a reasonable amount of time...
If interest wasn't a thing, it would take the exact same amount of time to pay off the loan as save the money. So the benefit of borrowing money is that you have now instead of in 20 years time.
But because interest is a thing, if you borrow money to fund the work it will actually cost you more money overall for the exact same thing. That is money you could otherwise have spent on yourself or your family going to the bank instead.
So if I were making this decision, these are the questions I would ask myself:
Is having the work done now rather than later worth the interest I will pay on the £40/50K, or would I rather keep that money to spend on something else?
Do I plan to pay it back by selling the house? If so, will it increase the sale price by as much or more than I borrowed including the interest? If not will I be ok having to lose some of the sale price?
Will the work I am doing now see me through for at least 20 years (or until I sell the house)? If not, what happens when I need work done again but I'm still paying off the loan for the last lot?