IANAL but this seems to be being done the wrong way round.
I assume the will splits the residual estate 80% and 20% to the 4 grandchildren.
If the house is the only asset, with equity release plus there are other debts I would expect the house to be sold by the executors. The money then used by the executors to repay the equity release and all debts. Then the residual estate (money) split 80%/20% as per the will.
There is no reason the house cannot be sold to a beneficiary and that is why you would typically get three valuations. As the op wants to “buy” the house they need to do this as a standard purchase. That is why @Sillymoo168 you need a conveyancing solicitor to “buy” the house from the estate.
If we assume the house is worth £250k, the equity release is £50k and the other debts are £20k. The residual estate would be £170k. 20% £34k is due to the Grandchildren and the remaining 80% £146k is the OPs 80%.
@Sillymoo168 you would therefore buy the house for £104k which is £250k less your £146k inheritance. The £104k would then be distributed as £50k to equity release, £20k to clear debts and £34k to the other beneficiaries.
From what @Sillymoo168 you have said you have paid the £50k equity release and the £34k 20% share to other beneficiaries. This means you still have to pay the estate the £20k to clear the debts. As part of this you then “buy” the house from the estate and this is why you need a conveyancing solicitor.
As I say IANAL but that is how I understand the situation.