my fathers british passport expired 20 years ago....

Please note that the validity of the passport is five years from the date of issuance or until all its pages are fully used, whichever happens earlier.
Once the passport’s expires the applicant can apply for a new one, as extension is not possible. 
Even if the Passport expired ages ago, like 20 years ago, it can be renewed but a copy of the Old passport have to be presented.
Documents needed to apply for a passport

When applying for a passport, the followong documents will be required to submit:

original and a copy of the Emirates ID card

original and a copy of the family book

a recent photo.

As stated before, while applying for renewal of the passport, the applicant must submit his/her old passport in addition to the above documents.

In the event that the intended question is otherwise, you can kindly ask us in detail and we will respond to it in the shortest time frame.