Hi Angie, I work for a mailing house and some of my clients are charities. I don't know what charity choice is as I've never come across it but I think I can help with gift aid.
If people are filling out donation forms then its worth including a gift aid declaration and tick box. If your wanting to target people who are regluar donators (by dd or whatever) or if they made a one off donation then you would need to write to them retrospectively. Do you need a hand with wording/what you have to include in the letter?
Hope this is helpful :D