Outright gifts to charities are totally exempt from Inheritance Tax.  All you need is to include in your will the words:

I hereby bequeath to the Governors of the Unicorn Preservation Society (Recognised Charity No. SCO 02771) the sum of £ __________ and the receipt of their treasurer or other proper officer for the time being shall be a good discharge for my executors.

For more information, please contact the Ship’s Manager.