Marmite website: Marmite Bestfoods
Navigating through the site, note that BestfoodsUK took over in 1990. This company is best known for its mayonaise (also Liptons, Knorr, Dove soap, etc.).
I looked at a package in the grocery and it was not imported from the UK. I can't remember whether the plant was in Canada or USA; I didn't buy it because I have been told that it is corrosive and is a substitute for acid in batteries (OK, I made that up).

Bestfoods, part of Unilever, is at: Unilever Bestfoods
Unilever: Unilever

Look through the divisions of this multinational British-Dutch corporation. Note that they have an advertising alliance with AOL-Time Warner. Products sold in the US and Canada are mostly, if not all, made in the US and Canada.