At which point they'd say "Yes, Watford is on the Met

"
Believe me if you went to Harrow, or indeed anywhere in London, talking about the Watford branch they
would think you are talking about the Met. I've never heard anyone describe Watford Junction as being any sort of branch line. People don't consider it to be some sort of branch, they look at it as being on the main line. People can't stand the slow trains and go for the fast trains where possible. Yes, they'd do HRW to WFJ on a stopper as that's not too bad, but if you see the number of people getting the fasts compared to the slows at Harrow, I don't think anyone gets the slows all the way to London, and from Watford you'd have to be insane.
In fact, if 2 London trains pull alongside at Harrow-on-the-Hill, you'll see loads of people rush from the 'slow' to the 'fast', even though there's not much difference on that route.