Charitable donations for previous years

peace

Was hoping someone could help with this: 

I make significant donations to charity every year
For several years I have not been a high rate taxpayer, so have not done anything. When I was a high rate tax year previously, I rang them up and told them how much I paid, sometimes they would ask for receipts, and I'd get a cheque back. 
This year, I'm going to be a high rate taxpayer. 
Is it possible to declare my donations from the past 3 years or so like you can do with professional subscriptions, or when you're working out pension annual allowance? Or is it just this current tax year? 

peace

and other question which I forgot to add: 

If for example, my predicted income is 110k. If I made a donation of 12k, my understanding is that would take me back down to the 40% tax rate (to miss the 100-125k 60% tax trap). Does that mean I would also get back 25% tax relief on that 12k?