I posted a while back on another thread about seeing a young Scandinavian guy get to the front at T5 to buy something like a bottle of water and they asked him for his BC - he didn't shout but said he'd "have to find it" - slowly unpacking his enormous carry-on rucksack one item at a time as the queue grew, and grew.... and grew....
Eventually the chechout girl siad "here - pay and just go"
If we make a habit of loading up the wire basket and getting to the checkout and then saying
"oh dear - I can't find my card - I'll just have to leave this basket here"
they might get the message