Hi Jolinsay,
Inheritance tax works in a slightly different way, presuming your parents are married or civil partners, when one of them dies, their estate will fall to the surviving spouse / civil partner and so will their inheritance tax threshhold.
When you lose your other parent, their estate will be subject to inheritance tax.
The value of the estate is measured at this time, with 2*£325000 inheritance tax threshhold applied.
If your parents own their own home and give it to you (and your siblings, including adopted, foster or stepchildren or grandchildren) the threshold is increased to £500000 and potentially a threshold of £1000000 (2*£500000).
If the second parent dies within 7 tax years of the cash gift, then it is included in the estate for inheritance tax purposes.
For more information, please have a look at the guidance at
How Inheritance Tax works: thresholds, rules and allowances.
Thank you.