If you pay UK income tax or capital gains tax, we can reclaim from the Government the basic rate of tax which you have already paid on the gift. This currently works out at 25p for every £1 you give.
If you are a higher rate tax payer, you can additionally claim higher tax relief via your Self Assessment tax return. This is the difference between the higher rate of tax and the basic rate. This reduces the cost to you of making the gift.
The HM Revenue and Customs website has more information about gift aid.