In the UK, you're pretty likely to get arrested for something stupid and given a criminal record if you do this. CCTV for the lose. It's everywhere - there are even CCTV cameras above the checkouts in supermarkets... I'd be willing to bet a tenner that those CCTV cameras can pick up people's PIN numbers as they type them in. Given how careless most people seem to be. Anyway, that's OT. Sorry. 'twould be funny, though. :)
I used to work retail security here in the use (Target). A store generally had somewhere between 150-200 security cameras, with up to half a dozen "command cameras" that were fully remote controlled from the security office. The zoom function on those cams was so good I could have easily closed in on someone's cellphone and watched them write a text message, so yeah, reading PIN numbers is child's play.
Isn't there a train station or some other location in London that has the most extensive CCTV system in the world? I read something about thousands of cameras? Can't even pick your nose without getting caught? ;)
If we wanted to do the sticker thing, the better approach would be to simply talk to a store manager in-person and get the OK. That's not difficult, and the pitch would essentially be "We'd like to stick an award on some of your products and help you sell more games."