I think you sound wonderful OP and wish you the very best of luck with your new 'look'. I think if it were me I might dip my toes in, so to speak, by not spending it all at once. I would DEF go to a top hair stylist and have colour and cut done. Revamp skincare and make-up. I love La Roche Posay for skin, you can get it in Boots and it is very nice without being stupidly expensive. Go to a beauty counter and explain the basic everyday look you would like, Bobbi Brown are generally very good. Buy the bare basics such as foundation etc and then you can add to it over time.
Browse nice shoe shops, try on LOADS of boots till you find the perfect pair.
Buy some GAP / Levis etc - I wouldn't go crazy for designer brand denim at this stage, what you want (I think!) is nice, well fitting, decent stuff you can wear EVERY DAY and not be afraid it is too expensive etc
M&S for cashmere jumpers / cardigans. Boden is good for this too.
Buy a couple of dresses from Boden, it is lovely to have something in the wardrobe for occassions where you might be going out for lunch / dinner at the weekend and you have a nice dress and shoes ready to go.
I would keep some of the money back so as confidence from the new hair / makeup / teeth / clothes etc grows you can add more luxurious touches to your existing new wardrobe.
If you can, spend time at the weekend 'people watching' - have a coffee / glass of wine somewhere busy and look for women a similar size / age to you who are dressed well to you and make a mental note so you know 'what' to look for the next time you are shopping.
Good luck!!! You deserve to feel fabulous!