If you do wish to donate to a charity then avoid the 'chuggers'. They are a relatively expensive means for a charity to gain subscribers. The charities deem it worthwhile because direct debits provide a comparatively reliable long-term income. I have heard estimates that suggest that the first year of payments by a donor who has signed up in this manner are used to cover the chuggers' fees. I can't confirm whether this is true or what proportion of the fundraisers are paid on comission.
There is a code of conduct which, although voluntary, most charities have signed up to. Details are at:
http://www.pfra.org.uk/cofc.htm?menuSet=down&s ectionSet=charities
I refuse to subsidise this method of fund raising and think twice about contributing, through any means, to charities who utilise it. If you do speak to a chugger who has been hired by a charity that you want to donate to, it seems to be better to organise the direct debit yourself online. I haven't any experience of people knocking on my door but this seems to be a particularly insidious method of garnering funds.
People do feel pressurised by this sort of in-your-face tactic and many people will sign up regardless of their ability to pay. (I don't want to generalise, but perhaps some elderly people may be more vulnerable to this approach).
Giving to charity is a matter of private conscience.