See a solicitor, as soon as you possibly can.
You need to consider the possibility that your DH is seriously depressed or not himself. A relative of mine had this sort of intention and had actually started writing cheques to various charities, at which point the spouse found out. (Actually it was the local priest who called to say he had received a large cheque from X and was Y aware, who tipped Y off).
I suggest getting your DH to sit on the money for at least a year, and in the meantime try to get him to see a doctor, preferably a mental health specialist.
My relative was eventually placed under a psychiatrist's care; the grandiosity inherent in the idea of personally saving the world by writing cheques was recognised as a symptom of a mental illness that was beginning to show itself. The spouse controlled their finances from then on. Most of the five charities that had received cheques returned the money, thank goodness, and the spouse sent them a token donation.
Your DH seems so adamant about this matter, and his thoughts are so completely devoid of any concern for his children and for you, his SAHM wife who will need to be supported into old age, and so unrelated to reality and the possibility of disaster (stroke rendering him both disabled and in need of care and unfit for work, job loss due to Brexit, etc) that I think a few sessions with a therapist might be needed. It seems to me that there is a strong emotional component to his decision that needs to be examined, because what he is planning is neither rational nor reasonable, nor even caring.