I don't think anyone except the older generation calls them underpants now, most everyone in America calls them underwear. I just call them what they are panties, boxers, tighty whities, etc. I use the word cardigan instead of sweater with buttons too. All the older generation calls pants britches where I'm from, which is the southern US. I've never heard anyone call flip-flops thongs either. I would have thought of them as a G-string too.
I've also noticed most British use the word bum like we do for butt. Bum to Americans means someone who is living off someone else i.e. "bumming off someone"
Here are a few I found on the web.
British: Rubber=Erase part on a pencil (which Americans just call an eraser)
American: Trunk= the back of the car where you usually store a spare tire, or put groceries
British say "arse", Americans say "ass"
"Belt up" means the same as "Shut up" to Americans.
"Blow me" means they are very excited to British, and to Americans...well it means to give a guy oral..or a blowjob as we call it.
"Blowing people off" to Americans means you stood them up, cancelled plans, or won't answer their calls...to British it means you passed gas on them. This one had me rolling.
"Blunt" is the same to British as "dull" is to Americans.
"blunt" to Americans means to be straight-forward and up-front about things.
Americans say "pissed off" British say "brassed off"
"cheeky" to British is the same as "lippy" or "being a smart ass" to Americans
"fancy" is to "british" as "desire" is to "american"
If British tell you to "keep your pecker up" they mean to "keep your chin up" to Americans, but we Americans would be thinking they were telling us to keep an erection. hahaha
"knob" means a man's genitals to the British, but we Americans might be known to tell them to "turn the knob" because we use this word for a door handle.