I had our administrative assistant at work metaphorically rolling around on the ground laughing last week. I made the mistake of saying that I needed to wear “pants with pockets” for a work task requiring a microphone with a battery pack; of course in her English way “pants” is underwear and the idea of them having pockets was apparently quite funny. I just cannot seem to remember to say “trousers.” I don’t get into trouble with “knickers” since I would be unlikely to use that word in conversation to mean anything but underwear (and I seem to associate the word with a certain Bridget Jones).
My favorite British word for underwear, however, is “smalls.” As far as I can tell it is used most frequently in the context of laundry–you can buy a “smalls dryer” which is just a hanger with twenty some hooks for grasping individual small items for air-drying. And unlike “pants” the word has no ambiguity about it; if I told an American I needed to wash my “smalls” they would be completely confounded.