A lot of the self-expression stuff seems to be understated but is definitely present. Things that you wear TO work but maybe take off AT work (gloves, scarves, sunglasses) are often extremely glam. You see a lot of hermes etc style silk scarves which is definitely a 'personal style' bit people add in.
The grooming standards are brutal for men, too. They get things 'done', a man-i-cure and moisturizer is fairly expected, and they definitely dye. The whole point is too look like you take care of yourself.
As I said, I teach in a very progressive (though madly high-powered) private school and my dress code is different (I don't dye my hair, for example, or get my nails done) but a similar understanding is under it all: We expect you to look like you take being here seriously, and that involves looking like you've brushed your hair.
Inside that people have wildly different styles. One woman only wears trousers, another is always in a very sophisticated Brooklyn-y pencil-skirt-and-cat's-eye makeup thingy... if anything, the men in their chinos are the blandest part. But it's not usual to show up with wet hair in a ponytail, or without a bit of makeup.
My outfits (knit dresses, knit blouses and flared skirts, variety of cardigans for our bloody freezing winter, opaque tights/leggings under dresses, lots of sweaters) are also a kind of uniform. I think we probably all have them, don't we? We find silhouettes that suit us, colours that makes us look good, and we mix them up and wear them over and over.
Or maybe that's just me :)