Discretionary Trust via a Deed of Variation by Siblings

AnotherJoe

I don't know the detailed answers but I do know you've got 2 years to get a DoV in place. 

Keep_pedalling

You can set up a charitable trust and qualify for the discount.

https://www.thepfs.org/learning-index/articles/charitable-trusts-and-wills/47446

Although you have 2 years to make a DoV, the IHT bill needs to be settled in 6 months.

xylophone

If we set up a charitable trust in his name, (placed the 15-20% of his estate in it and then continued his legacy of donating to worhty causes from this charitable trust), would that count as a charitable donation for IHT purposes ? (or is it only, if you give direct donations to existing charities) 

This is likely to be  costly, complicated and heavy on administration.

If you know that he had a particular interest in certain good causes, why not simply vary the will to make specific bequests to them?

Failing that, as the estate was left to you and your siblings, why don't you choose your favourite charities or good causes and vary the will to make specific bequests to them - to me that seems entirely reasonable.

https://www.gov.uk/donating-to-charity/leaving-gifts-to-charity-in-your-will