This is usually a punishment for children who have uttered taboo or blasephous words (the idea being that you said a dirty word and your mouth needs cleaning).
The other side being in reference to needing to litterally clean one's (dirty) mouth so that the taste of marmite can be savoured and enjoyed.
Condsider this JOKE EXPLAINED!!
You would then have vicious infighting between those who love, and those who hate marmite.
Oddly, can't stand marmite usually, however, twiglets are ridiculously tasty.
I'm entirely the other way round. Twiglets are a punishment meted out by an insane diety for consuming too many snacks in a former life.
Love 'em too (but don't often buy them as I've insufficient self-control to leave a packet half uneaten)! ;-)
Right after we finally decide who has the best hummus.
My departed mother and aunts loved the stuff but I'll eat it out of desperation/under sufferance when Vegemite isn't available. Where both products are available in a bousehold there's never-ending wars over which is best. That's almost inevitable. :-)
I made this “sandwich” with a crumpet, polish mayo (winiary), marmite and sometimes a scotch egg and/or sauerkraut. It was disgusting, I mostly created it b/c it was cheap, but it was a staple of mine and a very British one at that (aside from the kraut). The polish mayo is an import but I’d still say very london at this point, I got it from sainsburys, but could just as well have picked it up from any number of polish stores. Anyway, marmite’s alright.
True 'native' eaters like me spoon it straight from the jar! No problems.... ;-)
Edit: shame there's no image posting here or I'd demonstrate (squirm factor).
I think it has gone down a bit from the peak around 2016, but Poland became something like the third or fourth most common place of birth in the UK (after the UK, India and Pakistan I think) with roughly one million Polish immigrants.
[I have a Polish boyfriend living in the UK, so know too much about this ;-) ]
I like a thin layer of X-mite between a piece of toast and a fried egg. I find eggs need a lot of salt, so X-mite is a good combination.
But you are 101% correct.