Aren't they? (I mean, obviously not everyone, but I assume you mean particularly controversial like Marmite or the fermented fish products of the Nordics?) I (British) always thought 'sardines on toast' was pretty plain, standard fare. A step up from 'beans on toast' perhaps, but no less commonplace.
I'm an American who has traveled a fair bit outside the US. I'd never heard of sardines on toast until a British person made a thread about it on the cooking board on 4chan, maybe a year or so ago. I guess it's not too common in North America, at least in the stars-and-stripes part.
