Somr things do work as an abattement ie a rebate on tax paid. Which is how charity donations work I think. So for those, no tax paid means nothing to receive a rebate against for the donation.
Other things - like, I believe, the home help allowance - and some recent aides like eco energy things (eg eco boilers) will receive money reimbursed from the Impôts regardless of income. I guess it’s done by Impôts because until your tax has been calculated by them for the year they don’t know if you are above a ceiling RFR that wouldn’t be eligible to receive that money (all or part or none) paid back.
Sorry to hear about your wife and I can kind of understand your feeling that a little back from your donation might have helped you feel better - a kind of acknowledgement that it was for her.