There's some regulation decision in US that You can't put non-food in food. Quite a sane decision. Only that You can't have Kindersurprise in US. And if You bring one from outside, it will be confiscated.
Negotiating a mega-treaty with US will not be simple thing, as UK is aligned more on UE "style" than "US" style. On food for example, the approach is a bit opposite: US is rather "it wasn't proved as dangerous, so Yes" while UE is more "it wasn't proved as safe, so no".