Hair well cut, and if you colour it, keep on top of it.
Nails always perfect - if your nail varnish chips, take it off that evening, or at least re-do that finger.
Eyebrows are vital - have them properly threaded every couple of weeks and use dye (you can do it at home cheaply) AND powder (pencil can be too heavy) to get the right fullness and soft definition. I use Anastasia brow powder with an angled brush and it has been nothing short of transformative, particularly if you are fair.
Navy is a much under-estimated colour - invest in a good lightweight wool navy blazer (£250+) and have it altered to fit your perfectly. When I have tried on blazers in places that are just out of my price range, like Joseph or The Kooples, I have also wanted to weep at how much better cut and sturdier they are than their equivalent in Zara.
One of my best friends is short and busty, so she just takes it as a given that everything has to be altered to fit. She doesn't have to trawl the shops looking for a dress to fit her chest without being a tent any more - she buys the thing she wants, and takes it to a tailor. If you start treating that as the thing you HAVE to do, you can stop compromising.
Capuchine Safyurtlu is my fashion icon, and when I was working out what I would need to do to revamp my wardrobe more towards her look I looked her up on Pinterest/Google Images and wrote down all the common pieces:
Colours: Navy, black, white, grey, light blue, never ever pattern.
Shoes: black heels with pointy toes, generally always black everything
Accessories: black leather belt, about 1 inch wide, with EVERYTHING
Bag: small, never over-stuffed, usually black or white
Hair: natural, loose, casual
Lower half: black skinny jeans, cigarette pants, unless jeans, always hits at or just above ankle.
Makeup: natural
The belt thing was quite a revelation for me. (Samantha Cameron also does belts - she has little waist definition (like me!), and creates the illusion of one with skinny belts with everything she wears.) You can get really nice belts in H&M for about £5.99 to experiment with.
I'm not saying I now dress like a carbon copy of CS, but it's no harm to have ONE person with your sort of body type whose style you consistently admire when she is both dressed up AND casual, so you can focus on what she does, instead of just vaguely saying "smart wealthy women who I only ever see dressed for a royal, Wimbledon or G20-related event".
All the women you mention (as said above) are certainly polished, but much much older than you and/or renowned for their dull style. They are also tall and skinny, so there's no point emulating them if you are not also tall.