Well, speek'n as a truu-born Suvvenner, I DO know the difference between Watford & Watford Gap! (Watford is where the map turns all plain white with just a printed warning "Hereon be Dragons)!
Just for interest my Swiss wife HATES mushy peas and looks on with huge disgust as I happily devour a portion. But as I tell her, what does she know - them as what don't like 'em, there's just no hope for yers! Just like you lot ;-)
But being a little serious for a moment, I have the feeling (NOT knowledge) that mushy peas come originally from about the WWII time, when unless you grew them yourself, you were damn lucky to get any veggies unless they came out of a tin.
As said, SWMBO absolutely HATES mushy peas but she LOVES "duurbonnen" (not sure about the spelling without looking that up, sorry)!
What's that? Well despite being neutral in WWII, food, especially fresh food, was short and was rationed in Switzerland during WWII too, just like UK. As part of their Civil Defence regs, every Swiss family had to have so much food stuff - mainly dried - stored for "emergencies". Amongst other things that included flour, rice, AND the afore-said "duurbonnen" (which are dried green French beans BTW).
As a kid (she was born in 1941) that included said dried green beans (which, just for info, are soaked in slightly salted water overnight, then cooked in the much the same way as fresh beans are cooked).
Purely as guess work I guess that's the main reason she likes them so much, even still today, - because she had to eat them as a kid. So also as a guess, the reason I like mushy peas so much is for a similar reason? Dunno, but on the rare occasions I visit UK these days, one "must have" for me is a decent portion of fried cod n chips WITH mushy peas of course.
BTW, I have tried "duurbonnen" myself and must say they're no too bad at all - but nowhere near as good as mushy peas ;-)