Anything north of Watford is fair game

It's an affectionate bit of banter. But northerners do tend to bang on about some self agrandising twee bullshit about where they're from. Quite tedious. But in American terms the distance are miniscule

Southern English people are generally and widely recognised as the biggest twats in Britain. When this term is used toward Northern English people it is in desperation of southerners inner recognition of being a southern twat. / fin