Gift Aid is optional, and it is almost always worth using if you qualify. The charity claims basic-rate tax from HMRC on your gift (25p per £1 at current rates). You do not pay extra.
You must have paid at least as much tax as will be reclaimed on your donations that year. If you have not, do not tick the box — HMRC can recover the shortfall from you.
Higher-rate taxpayers can claim additional relief themselves via Self Assessment. That extra amount comes back to you unless you choose to pass it on.
Gift Aid does not change a Clarity Score and does not make an unverified appeal safe. Confirm the charity number first, then use the Gift Aid basics guide or the calculator if you want the arithmetic.