My favorite thing about 'pants' as a synonym for 'knickers' is that it's also an interjection. When you've put the washer on a hot wash with your favorite wool sweater, Pants! is a perfectly acceptable response. I find it useful at work these days, but mostly because I work with a group of young people who, rightly, point out my favorite interjections are terribly safe for work. Pants! isn't really either, but it is fun.
My least favorite thing about pants in England is best understood through a visual.