I was stopped twice on 2 different occasions coming into the UK back in 2011 and again in 2012.
The first time I was coming from Spain; I was randomly picked out of the line. They checked my Facebook profile and saw flyers advertising a party in London with my name on the flyer and a £15 cover charge. They claimed I made money without a work visa. I told them I made no money: it was for a benefit for cancer research. They checked back as far as 2006 on FB and saw "more" flyers with my name on it as a DJ and said it seems like you have been doing this for a while based on what we saw on FB. I got sent back to Spain.
The second time was similar, coming in from Spain, my connecting return flight was through the UK but on the next day, I would have to stay one day in London. Same thing: they sent me back to Spain because they thought I was coming into the UK to work (as a Disc Jockey).
I am going to Amsterdam this October. I have to make a connecting flight thru the UK. Will I get stopped while connecting from London to Amsterdam and when I go back to New York City?