Any UK taxpayer can make a Gift Aid declaration for the charity to claim. This must be done in writing and usually the charity will have a form for you to complete and sign.
You do need to ensure that you are paying enough tax to be eligible; your donation must not be more than four times what you have paid in tax for that tax year. If you were born after 5 April 1948 and your income is below £10,000 you will not be eligible for Gift Aid.
Read about the income tax personal allowance on the HMRC website