Vintage, that's funny about elf. I grew up here and for years thought inability to say th was a speech impediment, rather than the accent / affectation that it is.
Panties conjures images of little lacy garments, worn by young women in the 1960s. It is twee and faux sexy in a not very sexy way.
Pants is normal, or knickers but I bet some people don't like that. I'd say briefs are male, it is just the M&S packaging that says otherwise. Americans saying underwear for pants sounds to me as though they are so prudish as to lack a specific word for pants, so only able to refer to them in the wider context of undergarments.
You may not use knickers but do reference the, to us, far more archaic knickerbockers. To us, Knicks could only be an abbreviation for knickers!