If you're a UK tax payer then your donations to registered charities will qualify for tax relief meaning that an extra 28% will be added by the Inland Revenue for regular tax payers and 40% for higher tax payers.
The charity can claim back the tax you paid on your donations - that means almost another third being added at no extra cost to you.
There are so many places to donate I'm sure you've already done so but incase you aren't sure where to go and want the extra 28% to be donated then here are some sites that you could use - this is not a complete list:
Disasters Emergency Committee
British Red Cross