• You can change a person’s will after their death, as long as any beneficiaries left worse off by the changes agree.
• Any changes to the will must be completed within 2 years of the death.
• You can change a will to:
  - reduce the amount of Inheritance or Capital Gains Tax payable
  - provide for someone who was left out of the will
  - move the deceased’s assets into a trust
  - clear up any uncertainty over the will
• To change a will you’ll need to make a ‘variation’. You don’t need a formal document or deed - you can write a letter as long as it meets these conditions.

https://www.gov.uk/alter-a-will-after-a-death

The estate can pay Inheritance Tax at a reduced rate of 36% on some assets if you leave 10% or more of the ‘net value’ to charity in your will.
https://www.gov.uk/inheritance-tax

Where a variation increases a legacy, or includes a new legacy to charity, the charity must be notified by the taxpayer that assets are being redirected to it and evidence of this notification must be given to HMRC. But this does not mean that the charity has to sign the variation. Copies of an exchange of correspondence showing that the charity is aware of the legacy will be sufficient. If the charity is not told about the variation, then it cannot be backdated for tax purposes.
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/373615/IOV2.pdf