If the proceeds of the policy together with the £300 contribution are equal to or exceed the cost of the funeral there will be nothing to pay.
If there is an outstanding balance to pay for the agreed funeral arrangements, this will need to be paid by your representatives at the time of arranging the funeral.
If the proceeds of your plan together with the £300 contribution exceed the cost of the funeral, then the excess will be paid to your estate or nominated beneficiaries.