As the British Association for Shooting and Conservation Limited is not a registered charity, your Executors will, if your estate exceeds the threshold for inheritance tax, have to pay the due proportion of that tax on the gift. If the gift is not expressed to be free of tax the amount paid to BASC will be reduced by the inheritance tax payable. It is, however, possible for you to specify that the full amount of the gift should be paid free of tax itself so that the amount paid to BASC is not reduced and the inheritance tax on the gift (if any) is paid from the residue of your estate.
If you have any concerns about your inheritance tax position you can also discuss these with an independent financial adviser. They may be able to make recommendations to mitigate the problem.