How old are you?
As Houns pointed out, Reiss is great for slim frames and they have a sale on at the moment. I’ve had two Reiss coats over the years, both of which are still in my cupboard because I grew tired of them before they started looking tatty. Much better quality than you’d expect. Neither was more than £200.
Hugo Boss is always worth a look at this time of year. I’ve got HB wool/cashmere one which I picked up in the sale a couple of years ago for not much more than your budget and it’s a lovely coat.
Reiss can be a bit trendy, but they do tend to have at least a couple of “classic” jackets at any one time. Hugo Boss has a few more which are conformist, but still nice.
But, age and how you dress generally will dictate what you’ll get away with for coats. No point getting a slim and modern Reiss if you’re 55 and work in IT and plan on wearing a green suit under it.
But likewise, a boring and oversized coat from the more traditional outlets will be crap if you’re 30 and otherwise well heeled.
EDIT: just had a quick look on Reiss website and nothing looks interesting, but worth checking stores. HB wesbite offers more, but prices are generally cheaper in store by end of this month.