"He will look generous, but in fact it is despicable, because if you want to give to charity, you should do it whilst alive, so that you can take full credit of your own deprivation in return for recognition/good feeling etc etc"
He does this already.
Incidentally, if it weren't for bequests in wills etc, then very many charities in the UK would have to close down. Some charities receive up to 50% of their annual donations through wills. As a lot of charities in the UK are having to take over a lot of work the government used to do (think foodbanks, and healthcare research for example) because of the economic downturn, calling bequests "despicable" is an astonishing thing to do.
However, as an accountant, an ex-IFA and licensed will-writer, I would say it is better to leave a named amount of money to a charity rather than a percentage, as it is easier all-round. Otherwise you have to spend time working out the entire value of the estate before the bequest can be made, and it's shown to be very stressful for the families to have to do this at a time when they are grieving.
So, for example, instead of 10% of an unspecified total estate, leave £5k instead (or whatever the figure you had in mind was), because if there's not enough liquidity in the estate to cover the 10% figure when the final value of the total estate is arrived at, the charity can request that you sell things to cover the amount. For example, if you had a house worth £250,000 and no life insurance, and only £1,000 in the bank, the charity would be entitled to just over £25,000, (if you left 10% as per the OP) and where would the cash come from then? Another reason to get life insurance, btw.
A will is considered a binding legal document, and as a beneficiary, the charity is entitled to "their" share, just as any other person named in the will would be.
I don't call thinking of others after you're gone "despicable", but there is a lot more to think about when it comes to life insurance, and wills in particular than you think. See a professional (I don't write wills any more) as it's their job to help you see the things you don't ...