I once had to use a bathroom so stopped in a shop, they said customers only, so I bought the cheapest thing they had, a coke. Then, while I was in the bathroom

I once had to use a bathroom so stopped in a shop, they said customers only, so I bought the cheapest thing they had, a coke. Then, while I was in the bathroom another employee, who didn't see me purchase the drink started banging on the door yelling customers only. I looked like a tourist, this was in London near Hyde park, so maybe a bad area.