I think the customs thing is legit. I have known people stopped after they had passed through customs and were stopped outside the terminal. Their undeclared usually cigarettes were confiscated and a large fine was given . An official receipt was issued.A customs officer can and will stop you at any time if he believes you are smuggling or trying to avoid paying duty.
Another thing "the old chestnut" speeding and staying in the outside lane too long - are they not offences?
Sellers ripping you off? What does that mean? Trying to get as much as they can for their wares? Why not write to the British PM and get him to sort out the ice cream/hot-dog sellers in London.
