Is it possible to use tax in a previous tax year to cover Gift Aid?

Yes, it is possible to use tax in a previous tax year to cover Gift Aid

For example:

If you donated in 2019/20 but didn’t pay enough tax that year to cover Gift Aid, you can ask HMRC for the tax you paid in 2018/19 to cover it. You would usually need to do this after the end of the tax year in which you made the donations (so the total amount required is known).

You can do this by sending a simple email/letter to HMRC with the following:

Of the donations I made in the last tax year, I would like “x” pounds to be applied to the previous tax year.