I wouldn't worry about standing out, teens pretty much wear the same type of clothes on both sides of the pond. Other US brands are popular over here (Nike, etc.), so I don't they would look obviously American.
Yankees baseball caps are also very popular over here (not because everyone supports the Yankees, but it's the only MLB logo you can get in the UK LOL).
Just make sure they take care and use a bit of common sense (as they would in the US), like not flashing their mobile phones or large sums of money around.