I used to have a passion for fashion, but I think one I settled down with a guy that had no interest and also started spending my weekends hiking rather than going to the theatre, I kind of disconnected from that, worked far too hard, and gradually ended up looking a little sad and worn out. Suppose it can be a bit similar to the 'early years' of being a mum, people simply don't have the time and resources to look uber stylish.
I'd say don't buy a whole new wardrobe at once. Try on a few pieces, wear them, if you love something and it makes you feel great, keep it, live in it. If it sparks joy every single time you put it on, get some more similar styles.
Mine was an emerald Ralph Lauren faux wrap dress (faux wrap dresses may be a good place to start as they flatter most figures, especially if you're lucky to have hips or a bust). Got so many compliments in it and felt comfortable, so got a couple of similar items and built from there. Maybe one new item a month, but only something I completely loved, nothing so-so.
You could try out some maxi dresses, which can be really flattering and comfortable, and worn well with any footwear if you can't wear heels.
I'm guessing you have some of your old favourite clothes stored. Have a look at the/ try them on to remember what you used to love/ what you look great in. Can you source some similar pieces/ see what the brands you loved offer these days?
In terms of brands, I'd look at Hobbs and LK Bennett, perhaps Reiss, Joseph. You're guaranteed to find some statement pieces where you look at yourself and go 'wow', even though the overall vibe may feel a bit conservative browsing. Now, I don't suggest you get them at full price necessarily. Go to the shops if you like shopping in person, or get a big online order delivered to get an idea of your favourite styles, colours, materials. You can get some pretty decent deals on their sale. But usually, once I love a style/ colour and know my size, I look for it on Vinted, and very often find it.
So essentially, I wear pieces that are priced at GBP100-500, but I pay 10-40 for them. That's why for your budget, you could get around 10 very good, ethical pieces once you know what you want.
Here's an example of what brands offer with Harper in London: https://www.lkbennett.com/LKB-harper
In terms of websites, M&S and Next stock many brands these days, not just their own, so for example, if you're looking for a black skater dress, lots of options of different pricing pop up. It's also a good way to discover new brands. I mention those 2 as their online experience is super smooth, next day delivery, and returns get processed super quickly, too.
Being in London, I'd pop somewhere like Westfield for a day, as there's lots of varied brands under one roof, and it can again help you discover what you like, and see things in person. (Personally I hate in-person shopping as it's busy, pushy sales people with opinions, unflattering lightning, but if you're not sure what you like, it might be a good start)
Colours - really good idea about getting your colours done, or even just trying on different colours to see how they look/ feel. I used to live in black and beige, and they don't flatter me at all, whereas emerald or peach really suit me. Statement pieces often make an impact because the colour is bold yet flattering, right?
ChatGPT is also a great shout, just 'build a relationship' with it first if it makes sense. Mine's excellent at recommending clothes as it knows me so well.
Hair - is it generally in good shape? While it's not directly fashion, good hair and a generally groomed appearance lifts every outfit.
Autumn is coming so I'd perhaps look at blazers in all their forms as one purchase. May sound a bit old fashioned but hear me out :) Blazers come as blazer dresses, jackets, buttoned up, open, whatever. Lots of more fashion forward options available, e.g. leather trim, trendy sleeves, soft, more structured, colours etc. You can also accesorise them and wear them however you want.
There was a time when my body was changing a lot and I was doing a lot of public speaking, and blazers absolutely saved my life. The colours I chose really flattered me, I had to worry a lot less about what else I was wearing, as simple black trousers went with it well, and I could style them as more formal or quite trendy depending on the occassion, e.g. soft slouchy blazer with skinny black trousers, cool t-shirt and leather boots.
Looking stylish is of course great, but I'm nearly 40 now and strongly feel that I only look stylish if a) the clothes make me feel comfortable and 'like myself' b) I look in the mirror before heading out and think 'yup, definitely look fab'. I go back to old favourites a lot.
Personally I feel a small well curated capsule wardrobe, e.g. 3 dresses, 2 skirts, 2 trousers, 4-5 tops, 2 bags, 2-3 pairs of shoes, but all items that you absolutely adore and hope you never have to part with is an approach to workwear that can work really well.
I love watching stylish women come off the tube during London rush hour sometimes, nice to see people still love fashion and have fun with it.
Hope some of this helps and that you find joy in building your autumn work wardrobe again!