I'm starting to feel like a cheerleader for this app, and I'm not, honestly. But here I go again.
I suspect that the coffee shop didn't know how the app works Liz46. But in any case the QR code part is secondary.
The main purpose of the app is to identify if you have been within 2m of somebody for more than 15 minutes. If you go to the coffee shop and they take your details, they can trace you. But if you sit next to someone on the bus, you don't know their details so they can't be traced by name. But if you both use the app and then one of you tests positive, the other will get a message.
So, the app is in addition to the contact tracing done by pubs, coffee shops etc, and finds people you wouldn't be able to find otherwise.
I'm not sure where you got the 10% figure from Barmeyoldbat, but 10% would be better than nothing.