Off hand I'd say native candies/foods (for example, I saw Kinder Surprise on a podcast the other day but I've never seen them in America) and Doctor Who stuff. Some merchandise makes its way to the US but it can be pricy and I'm sure just scratches the surface of what's seen over there. Similarly I'd pick out other things which have niche interest here (soccer clubs for example) but which are hugely popular in the UK.

The Kinder eggs are considered an adulterated foodstuff and are banned from import into the US, so you do have to be careful about exports.

Ha ha. Touché! I expect it is mostly Marmite and Branston Pickle.