Yes, it's double bubble for high rate tax payers. I usually bump up my charitable donations by the extra amount I know I'll get back in my self assessment.
The Gift Aid the charity gets is the basic rate tax you will have paid. You then claim back another 20% as the higher rate element (assuming you're paying 40% on some of your income).
I wouldn't bother claiming the value of stuff a shop sold as a donation, you might open yourself up to investigation but I would enter all donations on your tax return.
https://www.gov.uk/donating-to-charity/gift-aid