I gave items to a Charity Shop to sell, and signed up for Gift Aid on the donated items when sold. This increases the profit to the charity, who send me an annual letter informing me how much Gift Aid was claimed. Should I put this on my Self-Assessment? Does it decrease my taxable income?
If you fill in a Self Assessment Tax Return then yes you should include that amount in the "one-off donations" total.
It will effectively decrease your taxable income if you are a higher rate tax payer, there are some worked examples on this page:
You donate £100 to charity - they claim Gift Aid to make your donation £125. You pay 40% tax so you can personally claim back £25.00 (£125 x 20%).
The way that this "claim back" is done is by adjusting the personal allowance to have the effect of making you pay £25 less tax.