I would think of a cause dear to you, and look for the charity that seems to support it the best. I agree that it is easier to see what goes on at local charities.
If children and families are a cause you embrace, you might look at Home-Start, which is one of the most effective and efficient charities that I know. It is a national charity that operates through local branches, so you could have a look to see if there is one nearby.
Having said that, some solicitors doing the wills for free, will nominate a charity for donations. As I understand, it is acceptable to say 'no thanks' and that you are donating to something else instead.