Thank you for your consideration. Many charities receive the bulk of their income from legacies of supporters who like to remember them in their wills.
If you would like to remember the GBSSG and have not made out your will, then it would be best to see a solicitor or other professional and give your instructions.
If you have made a will, then the simplist method of amending it would be to write a codicil, sign it in the presence of witnesses and keep it safe with your existing will.