I am a probate qualified chartered accountant.
When you die, your estate automatically has an allowance of £325,000 before any tax has to be paid. If your estate includes the family home which you are passing on to your children, then you gain an extra allowance which is the lower of the net value of the home, and £175,000. So if your home is worth more than £175,000 then you potentially have £500,000 of tax free estate.
If your spouse died before you, and left some or all of their estate to you, then you also gain the proportional amount of their estate - including the house - to pass on to your legatees.
When my father died, twenty-three years after my mother, we were able to use an extra 85% of her £325,000 allowance and all of the £175,000 family home allowance. So there was no inheritance tax to pay.
There are various other allowances too but those two are the ones which affect most people.