Fascinating replies, everyone! JFTR, my sister's style is definitely not boho, nor office-formal, it is dressy and modern. She only wears heels for special events, as, living in London, she walks a lot, and she is tall so tends to prefer low or flat heels for that reason too. She definitely wears her dresses hard: we once both bought the same (different colours but the same otherwise) silk-lined wool dress (yes, dry clean only), and a year later when I suggested she wear that dress to an event we were attending, she said that unfortunately she'd worn it into the ground and had to let it go. Mine is still looking good as new, some years later.
Actually, Suz, although your post reminded me of this issue, the posts I had in mind were on a fashion blog somewhere a while back, and the writer of the main post really did seem to feel a sense of loss thinking that she could no longer wear the clothes she loves and in which she feels comfortable. That does not seem to be how you feel, and of course I in no way intended to suggest that there is anything wrong with dressing in a casual style if that is what the wearer prefers. I just really felt for that poster and some of those commenting who shared that sense of loss, and I just wanted to encourage everyone to dress in their own preferred style, whatever that may be.
To those who would like to be more dressy but they live somewhere where everyone dresses in an ultra-casual way, such that deviating from that norm would attract unwanted attention or make them feel otherwise self-conscious, I should like to suggest that instead of just continuing to conform to that casual cultural norm, you might want to consider running a series of 'experiments' over an extended period.
I myself once experienced something like that when spending an extended period in a rural part of America (very different from London!), and I did feel as though people disapproved of my more dressy, fashionable style, but I'd feel a complete idiot dressing the way everyone there dressed, so that was not going to happen! But years later, I now know that that self-consciousness I felt must have been all in my own mind, because nowadays I dress exactly how I myself want to wherever I am, and I don't have any problem violating even the ultra casual dress code of parts of the world where I spend time.
So all I am saying is: IF you are a person who dresses one way but would rather dress another way, try dressing the way you want instead, and give it a really good try, not just for a week, but for, say, a whole season, before concluding that if you're in Casual City, California, say, You Must Wear Casual Jeans/Shorts Outfits or look out of place. And before you conclude that a casual lifestyle requires casual clothes, well, no one would describe my sister's dress as casual, but she definitely has a casual lifestyle, so IF you are a person who would really rather be wearing a more dressy look but you think you can't because your lifestyle is casual, consider trying harder to bring your style into line with your preferences rather than dressing in a way you aren't happy with.
Just a suggestion.
Sarah
P.S. My sister even bicycles in her dresses (with opaque leg-wear). She has not yet adopted the midi-length trend, preferring above-the-knee dresses.