Honestly …? If you genuinely expect to buy all those items new from shops (you don’t mention pre-owned) for £250 it’s likely you will continue to feel dissatisfied with your wardrobe. And you won’t build up any store of good things as you’ll be continuously replacing cheap stuff that has worn out.
Even in the sales, £50 per item for a size 18 is unlikely to get you the sort of quality you want to be able to look ‘cool’ but also like a grown up.
Firstly, assuming you already have some useable footwear, I’d say you want knee high boots rather than shoes. They make any outfit look more pulled together and sophisticated. There’s a limit to how much you can economise here as you need good, glossy leather in a contemporary shape. M & Spencer does do boots, but the better quality pairs cost the same as they do anywhere:
Dune boots
I would wait for the sales at Cos or Massimo Dutti, or similar places.
The boots would mean not buying another boring pair of jeans.
You are a bit late to be shopping for Christmas season. And I don’t know your style so no idea what sort of dress you’d like - but for decent quality you could browse
Plumo
Gather and See
Toast (maybe)
and hope for a sale in time for your events.
The coat you show would be fine for someone with other good coats - a wool overcoat for instance. As a main coat it would be a poor choice and would make your other clothes look cheap and a bit childish.
Generally, for basic items to boost a wardrobe I would start with
Community Clothing