As above, I'd start with your hair. Book a good cut and commit to having it cut regularly every 8 weeks through the year. Ask the hairdresser to advise you on any styling products and what sort of brush to use when drying/styling and make sure you have one and keep it beautifully clean.
I'd also think about some other aspects of your personal grooming. Even if you don't wear nail varnish, your fingernails should be well shaped, with neat cuticles, clean and buffed and your hands well moisturised.
Make sure your feet are well looked after (no hard skin on heels), nails neatly cut and polished if you like.
Waxing/hair removal of choice done regularly.
Daily shower and use a really nice fragrance you like (doesn't have to be mega expensive).
You don't have to wear make up to look well groomed (I know several women who don't) but if you do decide to, remember less is more. I went for a total refresh on my make up in my mid 40s (I was terrified of being in a rut with it) and had a fabulous makeover on the Bobbi Brown counter at our local John Lewis. Picked up some really useful ideas and advice on application and no heavy sales techniques.
Why not think about a good bag for work and some decent shoes? Not size dependent and again, they can lift your whole look remarkably without shelling out a fortune.
While you are losing weight, use the time to do some virtual shopping and begin to narrow down your favourite stores/looks. I'd practice for instance using Pinterest and set up your ideal spring wardrobe on a board - you could feature some workwear, plus some outfits for your social/relaxation time. You could also consider having your colours done, so you can understand what colours really suit you and begin to build your wardrobe based on those.
When I used to work in an office (am now WFH), I kept a bag in my desk drawer, and every lunchtime I'd make a point of nipping off to the ladies and cleaning my teeth, brushing my hair, checking and if necessary reapplying any makeup, quick squirt of anti perspirant, fragrance, and after washing hands, hand cream. I also kept things like a spare nail file, tissues, wet wipes (useful for so many things!), hairspray, and a cooling foot spray. That way, I was always fresh and presentable for afternoon meetings etc.
Finally, my best anti frump idea was to go through my undies and nightwear. I ruthlessly discarded anything greying, saggy or baggy and over the past couple of years I've restocked so now have a lovely range of bras, knickers and nightwear that are pretty, comfortable and gorgeous colours, so even if I'm wearing jeans and T shirt, underneath I always look and feel happy. It's amazing the lift that and good fragrance can give you.
Well done on your brilliant weight loss so far and the promotion. 