You're certainly not the only larger woman whilst I'm around !
As I said, I get my staples from M and S Autograph and Boden. Also sometimes from Poetry and Wrap online (Poetry go up to 24, Wrap to 22) and occasionally Monsoon.
Was going to recommend Marina Rinaldi if budget not a problem. Have bought loads of things from them in my time. Unfortunately, not a very useful online presence, and in UK just one shop in London, Bond St. and one in Edinburgh. They are actually Italian - the plus size line of Max Mara, with the quality that goes with that. Some of their stuff is very staid/ conservative - though I have got great office wear, black trousers, etc from them. But I like to seek out their funkier stuff - mainly to be found in their Marina Sport collection. Have to say, do tend to get most when I'm in Italy as their stores in Milan (the flagship) and Rome are bigger and have a wider choice of items. And their clothes are cheaper there too. But realise that may not be very helpful if you're not going there soon.
Have also got a few things from Elena Miro - again Italian, worth a glance if in Italy, but it's not generally to my taste - a bit frumpy, like so much plus size stuff. Nor is Anna Scholz - too many lurid patterns and often a bit 'off' fashion-wise, and very £££. Do have an OK pair of their denim jeggings, though.
And (though this may shock some S and Bers)I do have and like some NYDJ jeans - not at all 'mum jeans' looking - what they call jeggings but I'd call skinnies. They're my everyday uniform, with tunics, lonline knits, etc.
Evans, Simply Be, New Look aren't for me - prefer better quality. Have never found anything I like at Ulla Popken. Land's End OK for genuine 'utility wear' but otherwise not really stylish enough. The Danish online company Bon'a Parte are worth a look, though.
Also,some Isabella Oliver things go up to 20, though not cheap.
I know what Buda means about loose layered look - sure it looks great on her and others who get it right, but personally I don't like loose things on me - think a ship in full sail - and think they make most - not all - large women look larger. I want definition for my waist (have an hourglass shape) and jeans/ trousers that fit pretty tight to my legs, as they're long and slim. Sometimes can just get the deinition for myself, eg by pairing an Autograph tunic with a Boden belt (they do good belts in an XXL which actually is XXL).
Generally (except for Marina Rinaldi) prefer to stick with 'ordinary' mid-market brands that go up to 22 rather than 'special' plus ones, as so few seem to get it right.
Sorry for rant/ stream of consciousness. Good hunting!