Bobby, I don't know your circumstances, but here's a made up example of how it works that shows how few people are going to end up paying inheritance tax.
Jack and Mary are married and jointly own a house. When Jack dies the house and all his other assets pass to Mary. No inheritance tax is ever charged on what one spouse leaves another, so Jack's inheritance tax allowance isn't used.
When Mary dies everything she owns passes to her daughter Lucy. The national average for UK house prices as of December 2023 was £285000. Let's assume Jack and Mary's house was worth a lot more than that - £600000. Let's also assume that Mary had some savings - £20000. So total estate is £620000.
There will be no inheritance tax on this estate, even though that's al ot of money for Lucy to inherit.
This is because firstly we have Jack's unused inheritance tax threshold - £325000 - which is added on to Mary's £325000 because it wasn't used when Jack died. So that's £650000 straight away, and that's more than the total value of the estate.
If the house had been worth a lot more, another allowance would have helped too. Each parent gets an additional £175000 that gets added onto the £325000 each if they leave their home to their children or grandchildren. So that's £500000 - half a million pounds - for each parent to leave their direct descendants free of tax.
Even if there was inheritance tax to pay, it's not on the whole value of the estate. It's 40% of the amount above the threshold. So a family like the one above can pass on £1m with no tax to pay. But even if we up the value of the house to £1m, using up the entire tax-free allowance, there's only going to be £8000 tax to pay (40% of the £20000 savings).
Obviously if your Mum isn't going to have unused threshold from a spouse, that changes the position. But her assets have to be very substantial before you need to worry about paying any tax.
This webpage explains how to apply to postpone paying inheritance tax because you don't have access to any funds before you get probate. I think you might end up paying interest when you do eventually settle up, but HMRC aren't going to foreclose on you if you can't pay up within the first few months after your Mum's death.