Bravissimo is fantastic for proper underwear and the team there are AMAZING at making it a fun, comfortable experience for teens too. Why don’t you go and do it together?
All clothes look better with good support underneath. So do that first and then go shopping. Chat about what she likes and why she likes it - if you can get to the feelings behind what appeals about a certain look, then you can find something that will work for her body shape to deliver the same effect.
What colours, fabrics, images is she drawn to? Can she build a Pinterest mood board of things she likes? And one of things she hates and wouldn’t be seen dead in? It will open the conversation and will give you more information to go on when you are shopping together.
Also, don’t overlook the other style stuff that might make her feel good - footwear, accessories, jewellery, make up, scent, haircut. The power of a timely glow up can be pretty powerful taken all together…
In terms of shops:
New Look (good range of sizes)
M&S (especially their excellent wide legged jeans)
Simply Be
Next (sometimes)
Band T Shirts can be a way to feel cool and connected without putting on a scrappy, body con dress
And possibly explore Vinted together?
Good luck. Brilliant that you are supporting her to feel her healthy, strong best. She will remember and be grateful all of her life. It’s hard being a bigger teen when you have a load of tiny mates. By finding her special, lovely things that make her look and feel good you are boosting her self esteem and showing her that she deserves it. This will encourage her to do the same for herself as an adult. Priceless.