Another vote for Ilse Jacobsen here.
As soon as I saw you wanted size 18, I knew I had to share this.
Its not cheap, this one is discounted at £165. But by god it's worth it.
I made it a mission one year to get a one-off investment coat. I have never spent close to this money on anything else.
I tried on every single coat in Joules, Barbour, Seasalt, White Stuff etc. They all tented me. But the Ilse Jacobsen comes with a slim fitted shape with boobs, waist and hips, and is in a super flattering, soft, comfortable fabric. It was way above the rest for me. I'm a top heavy size 18 so I got the 20.
The beauty of it is, it has a fleece lining so warm enough, but also goes over cardigans and jumpers if needed.
I cannot recommend highly enough.