I just finished watching this and, oh boy, what a good adaptation. Probably safe to assume this is set when the novel came out, so it's aiming for 1933-34. The costume designer was Laura Smith, who also worked on Emma. (2020) and Murder on the Orient Express (2017). Overall this miniseries had beautiful costumes with plenty of high quality reproduction items mixed in with genuine vintage pieces. This is a bit outside of my favoured decades, but with my limited knowledge as commentary, here are some of the sartorial highlights and some vintage patterns available online that would help recreate them:
... or generally pointed collars with wide spread. Perfect.
Casual menswear looks including lots of knitwear
I love the colour palette of these looks and it's so nice to see male costumes that aren't just suits. There are some good, slightly later shirt patterns at the vintage pattern shop, and good knit vest patterns on Etsy from sewmuchfrippery and various others.
Sidenote: Is there anything more attractive than fishtail back trousers and wide striped braces? I don't think so.
Also, Knocker wears tab/grandpa collars without the changeable collar as a casual look. This is a really good workman-style look and something I've been thinking about a lot recently and trying to find photographs of. Would people in the 30s think it was inappropriately underdressed outside a workroom, or cool? Who knows?
Good ladies knits
This jumper is haunting me, I know I've seen it before and I'll update this once I find it, but this Stitchcraft pattern from 1937 is the closest I can find for now. Possibly it was an Emmy repro knit once upon a time? I don't know.
There was also a lovely cameo by the Peggy Sue Cardigan by Emmy Designs in peach.
Floral wrap dress
This is a style of dress I've been meaning to make for a while so I screenshotted it to death. The flutter sleeves and wrap closure are perfect for summer. I think the design looks very simple for the 1930s, most sheer dresses in magazines from that time had more frills and fussy details. It might have been done to appeal to a modern audience, but also Frankie is quite a no nonsense lady, so if it is an artistic choice, I think it fits the character and is well justified. There certainly are simpler extant garments from that time, but to have the combo of flutter sleeves, no collar, and a full(ish) skirt is not something I've seen outside of nightwear in the 1930s. Combining these patterns (Wearing History, My Vintage Wish) would get you close with a chemise or full slip underneath.
The Closet Historian did a great post about 30s/40s wrap dresses and she's generally brilliant for drafting advice.
White Tennis Dress
I love old sportswear for women, it's detailed and infinitely practical. This looks a lot like New York 1040, but this Reconstructing History pattern could get you close. I think the skirt might be detachable but I'm not sure as it's already quite short.
Repro Metallic Heels
I love spotting reproduction vintage shoes, and I seem to have developed something of an encyclopaedic knowledge of them from my dancing life. Most of the film is shot quite close so we didn't see a lot of shoes, these two were both prominently featured. The silver ones are Ritz by Remix, and the gold ones are Susan by Memery.
Trousers for Frankie
I loved Frankie's looks throughout the film. It felt like when she was being herself she wore trousers, and when she was trying to blend in with society she wore dresses. The House of Foxy and Emmy Designs have been making really good reproductions of these styles.
Frankie wore two excellent cape-esque things, one was a burgundy coat and skirt set, and the cape joined the back of the coat over the shoulder blades. This colour is perfection and very true to a lot of fashion plates from these years. The other cape is a faux caplete attached to the jacket of a black skirt suit with shiny black buttons. Both of these are probably best to just draft from scratch with your favourite skirt suit pattern.
Olive Green Evening Gown
This is lovely and so simple in it's cut, and the dress clips make it exactly the right level of fancy. I love evening wear that covers your shoulders so you don't have to make matching shawls. You could get close with something like Reconstructing History 1314, McCalls 8118, Lady Marlowe 6779, and sleeve 2 from Vintage Pattern Girl T2149.