I'd query why you want to look "posher" op but each to their own!
Model a Parisian woman.
Good quality, carefully chosen accessories help like a leather belt with a buckle that matches either gold or silver jewellery.
A good haircut that is smooth and low key but well cut and glossy and off your face. Not too "red" or dyed but more low key ash tones depending on your natural colour but try and stick to as natural as possible.
Some plain, classic earrings and maybe a discreet necklace and watch.
Good quality sunglasses and handbag.
Stick to three classic colours like: stone or tan, navy or black, and white.
Maybe a silk scarf? Or a light cashmere jumper around your shoulders?
Classic leather shoes that have lowish heels?
Have clothes altered to fit.
Agree about changing high street buttons for something better quality.
Pale pink well manicured nails, or French manicure.
Jeans that really fit well.
Natural discreet make up.
Everything ironed, repaired, clean, neat, polished and considered (unlike me who gets dressed in 5 mins flat without looking in the mirror).
Appropriate clothes for weather and location. Nothing too frilly or flouncy or shiny but low key and well fitted.
No obvious logos.