I always considered that miles for charity was utter nonsense no matter which airline or hotel chain offered that option. I say that because you actually trade in a large amount of miles and/or points in exchange for a -relatively- very small cash donation on behalf of the airline/hotel. It simply isn't worth it, especially when one thinks what you can get with your 100,000miles.
Under the circumstances I can only suggest two things:
a) Give your miles to a friend, a relative or someone from our board, who will for sure make good use of them, as it would be a pity to leave all these miles in your account if you don't plan to fly BA again.
b) As was previously mentioned, give these miles to whomever you want and ask them to make a donation to a charity that you want on your behalf.