I've been using this app for quite a few months now and it's very good.
You buy a gift card and get a % of the money spent back into the app immediately.
As an example, I'm doing a Tesco shop today so bought a £50 card and they've deposited £2.05 into my account.
Loads of places are included, like Asda, John Lewis, Aldi.
After a while, it really adds up.
If you're interested, I'd be grateful if you use my referral code when you sign up, and I believe we both get some points:
QNEZ