I can tell you what my son likes, he has many favorites that we MUST get whenever we go to visit the Aunties.
Chocolate is at the top of the list. His personal fave is Aero bars, and also
Flake, of course. He also likes McVitties biscuits, several varieties, but especially Penguins as he loves the jokes. Cheddar and onion chips are good, too, though hard to carry -- that flavour is essentially unknown in the US.
Books: My son's Horrible Histories collection endlessly fascinates his friends. UK editions of Harry Potter are fun for fans. For little children, the Mr. Men series are great.
Teens often like t-shirts, bags, etc. with the London Underground logo on.