I've had so many attempted fake notes in Heathrow, last week had 3 guys with what looked like the same batch of fake notes within days of each other. There are a lot going around in Heathrow, anyone else visiting should be careful to check all notes, some are really convincing at first look but if you get caught with then in a shop the police get called (I told them they came from a currency exchange. They took my details on the spot and left but I imagine if it happens again there will be questions) you can get a cheap light or pens from amazon