If your charity has events where you accept donations you should be reclaiming gift aid from HMRC using the Gift Aid Small Donations Scheme GASDS. The maximum donation for which you can make a claim on the GASDS scheme is currently £20 but due to rise to £30 in April 2019, and it's important to note that donations are not a membership or ticket fee
The rules are straightforward, you must already be registered for gift aid and have made a gift aid claim. Your charity can then claim a payment equivalent to gift aid on cash donations of £20 or less, without the need for the donor to complete any paperwork, or for the donor to be a taxpayer. GASDS Claims are made alongside Gift Aid claims using the Charities Online template provided by HMRC. Note that an individual donation can only be eligible for one type of claim. It is either eligible under Gift Aid or the GASDS but not both.
HMRC states that to make a GASDS claim you must have claimed Gift Aid in the same tax year as you want to claim GASDS and without incurring a penalty in the last two years. You must also have claimed Gift Aid in at least two of the last four tax years (without a two-year gap between claims). Your GASDS claim can’t be more than 10 times your Gift Aid claim – so if you receive £100 in Gift Aid donations in a tax year, you can only claim on up to £1,000 of small donations under GASDS for that year (when you actually submit the claim is irrelevant). The maximum GASDS claim is £2000.
I found a really good example on the stewardship.org.uk website:
A charity receives £8,800 in a tax year in eligible cash donations with documented Gift Aid donations of £800 and small cash donations of £8,000. The charity can make claims on all of its donations; £800 under the Gift Aid scheme, and £8,000 under the GASDS because the matching principle allows for a GASDS claim of ten times that of the Gift Aid claim for donations received in the same tax year. The charity will receive £2000 in GASDS (25% of £8000) which is currently the maximum and £200 (25% of £800) in Gift Aid from their £8800 donations.
Good to note that you should inform your supporters that they need to commit to Gift Aid to allow you to make a GASDS claim.