There are signs at all the check-ins at Airports about what should be put into the plastic bags before you go through the x-ray machines. The signs have been there for years. I have always been asked are there any liquids in my handbag at Aberdeen, Edinburgh and Glasgow.
We watched a young couple at Aberdeen having to open a bag where they had two bottles of milk for their baby, the Customs made them drink from one of them.
About paying... They have always made you pay 50 cents for a bag in New York Airports, so British Airports are maybe following suit and making profits before the ban is lifted next year.