
The simplest way to give is via 'Gift Aid'. This allows charities to claw back your tax from the Inland Revenue on either one-off or regular donations. There's no minimum contribution so even on a pound the tax is reclaimable.
All the charity needs is your name, address and a declaration that you're a UK taxpayer. As this can be verbal it can also be done over the phone.
How does this benefit you and the charity?
Charities reclaim the tax at the basic 20% rate, which due to the way the numbers work means they get 25% more than you donate (so if you give £10, the charity gets £12.50).
Any higher rate (40%) or additional rate (45%) taxpayers are able to reclaim the difference between the basic and higher rates (ie, 20 or 25%) on top of this.
For higher-rate taxpayers, on £10 that's another £2.50 (so £5 in total), and for additional rate taxpayers, it's a further £3.12 (so additional rate taxpayers can give £15.62 from a donation of £10 - 56.2% on top of your original donation).
Higher and additional rate taxpayers can choose whether to claim the extra tax relief on the tax self-assessment form, though there's also an option to donate this extra tax to charity too.
To claim that cash, either declare it on your tax return, or if you don't fill one out, complete a P810 form, available from your local tax office.