I believe that any money held in a Sipp at the date of death is IHT free, so outside of your estate, as long as you haven’t started accessing it.
https://www.youinvest.co.uk/pensions-and-retirement/accessing-your-pension/sipps-and-death
SIPPs and death
One of the great tax advantages of a Self-invested personal pension or SIPP is that they allow you to pass on your pension to your beneficiaries on your death. Your beneficiaries can normally choose to take the pension fund as a lump sum or leave it invested in a SIPP.
What happens to my SIPP when I die?
You can nominate whoever you like to receive your SIPP on your death. This could be your spouse, children or grandchildren, or you can nominate someone unrelated to you if you wish. You can also leave some, or all, of your SIPP to charity.
You do not need to leave the benefits to just one person, you can split them in whatever proportion you like, so each of your beneficiaries receives a share of your SIPP. To inform us of your wishes you should complete a SIPP death benefit nomination and expression of wishes form.
Sorry, I’ve got a bit hung up on SIPPs here. Other pensions probably differ.