I think the problem is that different companies, sectors, etc. are all so completely different.
I've worked in and around the city for years and over those (25ish) years have gone from full on skirt suit with heels - pinstripes with white shirt etc. Through to smart skirt/trousers and smart top, and now where I work it's pretty much jeans or a "cool" outfit (I find the "cool" bit very difficult to figure out!). Men have gone from suit with tie, to suit without tie, to trousers with shirt.
The younger women are sometimes wearing stuff which might've seen them disciplined 25 years ago - ranging from micro skirts, to see through lace, to all sorts. But not in more senior roles.
Probably the best is to try and find some photos of people who work for your company, and see what they're wearing.
I've never seen any issues with sleeveless and the longer dress with the interesting neckline would be appropriate although chilly most of the year (even in the summer with air con on) without a jacket or something with sleeves over the top. But then I haven't worked in the legal sector, but from what I know of a friend who works for one of the big city firms, they're getting much more relaxed these days.
I've only been back in a few days here and there since Covid, maybe things have changed again, but walking around the City you see such a wide variety of outfits from mega super smart, to informal and funky.
Hard isn't it !