Don't recommend Anna Scholz - ridiculously expensive and most of their stuff - except for a very few pieces - is not even stylish, but just a bit weird/ in glaring patterns and unflattering styles, eg cowl necks.
I don't personally like any of the 'specialist' online retailers - Evans, Simply Be, Marisota, etc - either not stylish or not good enough quality, IMO. But that's a personal thing.
I get most of my stuff from Boden, who go up to 22. You have to pick and choose carefully (and avoid garish 'fun' patterns) - but you have to do that anywhere. Unlike you, I'm tall as well as large, but the reviews on their site usually give a good idea which styles suit taller or shorter people.
Used to get some basics from M and S Autograph but they seem to have generally gone down a bit recently - too many Per Una-ish things. There are a few good plain basics there, though, if you look really, really hard.
I love Hobbs - can't quite get into most of their stuff myself, but I expect you can.
You could also definitely wear Monsoon - they go up to 22.
Good hunting.
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