I expect the reason they are packed in China is cost and convenience.
Cost is wages, packaging materials, power, property costs, taxes, transport, waste disposal (they need more Arches!) and a zillion little things I can't think of, all cheaper there.
Convenience: biscuits from UK, coffee from (say) Kenya, Brazil, Vietnam. To be sold in: UK (as BHS), France, Russia, USA, Australia, Brazil, Kenya, Vietnam, Greenland. All has to come together in one place, and China has the labour, the materials and the links to do it best, on average, for the whole world.
That, at least, is what I guess. I'm not saying it's necessarily a good thing, just that it does make sense in its own way.