I'm not dwelling further in English underwear, but on British slang. Found a fun little book, Dictionary of British Slang, by Jonathan Bernstein, 2006:
bunk off -- to play hooky
do a runner -- leave a cafe without paying
wobbly -- refers to somebody untrustworthy
the trouser department -- referring to male genitals
braces -- suspenders
glossies -- magazines
corker -- an enthusiastic opinion
panda car -- small police car
gammy -- the injured part of the body
twit -- idiotic behavior
paddy -- catchall term for Irishmen
poxy -- most unpleasant
number plate -- car license plate
dustman -- garbage collector
lollipopman -- crossing guard
brolly -- umbrella
that takes the biscuit -- that beats anything I've ever heard
tickety boo -- everything's going great
Please feel free to employ any of these in your daily dialogs.