As another poster has mentioned, most, if not all, charities are only too happy to reclaim the 28% tax back and you can normally give the charity permission to do this by whatever means of donation - the only exception I can think of is collections in the street (e.g. with buckets) where the tax isn't reclaimed due to the logistical complexities involved, although I was at a Tom Robinson concert and he (and his organisers/helpers) provided appropriately marked-up envelopes  for everyone to put their money in before being put into buckets for a charity collection.
Donation by credit card is a more expensive (administration-wise, due to credit card charges which charge a percentage, just as they do for shop transactions) method of donation than debit card (which charge a low flat fee), cheque (fees depend on the bank that the charity is with) or cash (this also has cost involved as it takes administration time to count and bank!). I suspect Direct Debit directly to the charity is probably the best method of payment as it is low cost, straight to the recipient bank account and additionally gives the charity concerned an almost guaranteed income stream, whereas all the other methods tend to generate a far more variable income stream. However it's better to give via any method than not give at all!
Alternatively there is online schemes like http://www.allaboutgiving.org and slightly more usefully http://www.allaboutgiving.org//static/charitycard_uk_aboutcharityaccount . aspx and they take out a set amount of money, via 1 Direct Debit, out of my bank account each month. It's then up to me to decide which charities to donate to (they have a search engine covering all registered charities in the UK) and how much I want to donate to each one and when I donate, otherwise the money just accumulates; I find this easiest as I can be consistent in how much I donate over the course of a year, and I have an online record saying how much I have previously donated. They also take a (small, stated, and taken from the tax automatically reclaimed) amount out of the money donated to cover their admin costs. I believe there are other similar schemes available.
 the envelopes had marked prompts for the gift aid declaration, your name and address
Typing from Barking