Do others sometimes feel that they've over-formalised, and had to live with a totally out of place look for a few hours?
No, I can't remember the last time that happened (It'd be just as embarassing to show up for a formal event "over-casualized"). I always strive to dress appropriate to the occasion, and if I ever have a question regarding the level of dress expected, I ask. It's really no more complex than Sassenach dress...actually it's somewhat easier; with a kilt I have the direct equivalent of blue jeans, slacks & a blazer, a conservative business suit, or a tuxedo all in one garment. It's simply a matter of knowing how to dress properly.