What you are asking if for your parents to increase the mortgage on their house by £80k.
Firstly, after their existing mortgage, there must be the value in the house in order to secure that amount of loan. You must be able to afford the repayments and consider the length of the term of the mortgage. If you do not pay your parents (who will have to make the repayments themselves) the amount, then they will be liable.
It's a mortgage - go to any bank or building society website and play with their calculators.
The important thing is that your parents' house has sufficient value to be able to cover the mortgage should you/they default and secondly, you can afford the repayments. I hope your father (and you!) appreciate the generosity of the gesture
Cheers
Whirls