If you were given a gift of the �80,000 - rather than a repayable loan - then the �80,000 does not form part of the estate, except for the particular purpose of determining the extent of any IHT payable.
For example, suppose your father had given me the �80,000 as a gift, instead of to you. I am not a mentioned beneficiary under his will, but I still have the �80,000 and the value of the assets available to the named beneficiaries is therefore �80,000 less that it otherwise would have been, and any IHT due on the gift to me would have to be borne by the beneficiaries of the estate.
The point I am trying to make is that your share of the estate is whatever your father's will says it is. The �80,000 would only be taken into account if your father's will said that your share should be adjusted to take account of the fact that you have already received the gift.