On an estate of that size I would be very surprised if there was no will in place. A substantial amount of inheritance tax will apply.
Overview ( from gov.uk )
Inheritance Tax is a tax on the estate (the property, money and possessions) of someone who’s died.
There’s normally no Inheritance Tax to pay if either:
the value of your estate is below the £325,000 threshold
you leave everything above the £325,000 threshold to your spouse, civil partner, a charity or a community amateur sports club
If the estate’s value is below the threshold you’ll still need to report it to HMRC.
If you give away your home to your children (including adopted, foster or stepchildren) or grandchildren your threshold can increase to £500,000.
If you’re married or in a civil partnership and your estate is worth less than your threshold, any unused threshold can be added to your partner’s threshold when you die. This means their threshold can be as much as £1 million.
Inheritance Tax rates
The standard Inheritance Tax rate is 40%. It’s only charged on the part of your estate that’s above the threshold.
Example
Your estate is worth £500,000 and your tax-free threshold is £325,000. The Inheritance Tax charged will be 40% of £175,000 (£500,000 minus £325,000).