Just had a chance to read over previous comments. Some good advice there.
Agree there's probably little point in a personal shopper,as I've never yet found (and, believe me, I've looked) a department store that has a reasonable slection of plus size clothes which are not cheap and nasty. Harrods have a recently much reduced (in size) dept., but they have also made the stock (even more) dull and frumpy. Guess you might find a plain, dark Marina Rinaldi suit there if lucky. Some stores stock Chesca, which are hideous, and not work-friendly.
On bottoms, it would be worth having a look at Boden, who now do up to 22, and have some wool trousers. Could be worn with a jacket from elsewhere, eg Marina Rinaldi.
M and S Autograph trousers may also be worth exploring - I've got quite a few of their dresses, which go to 22.
On 1647, they are truly horrendous - one of the best arguments for weight-loss around, though Evans do quite well in that respect too.
On Spirito di Artigiano, interested to see it recommended and in what you think of them. I've bought some basics there in the past and there are still a few relatively innocuous T-shirts/ under-pieces on the site. But they have, IMHO, become ever more frumpy and now stock mainly the sort of things I try to avoid, eg trousers which are far too baggy, and vast shapeless 'over-size' jumpers. Like others, I think definition, not 'ship in full sail effect' is needed. They also have pull-on tousers ...
Poetry and Wrap have some nice things and go up to 24 and 22 respectively - have bought a couple of pretty summer tunics from them. But think they are mostly casual/ floaty rather than work-y.
Anna Scholz have great PR and a lot of press coverage but, on the few occasions I've ordered something from there (their hectic bright prints and would-be but aren't quite funky things not really to my taste) have returned them. The quality is not good,in my experience, despite the price. If you're spending that money, Marina Rinaldi a safer bet as quality is excellent. You could probably afford to fly to Italy (where Rinaldi cheaper) as well on AS prices !
Someone mentioned underpinning. Sure you already know the value of a great bra. Really recommend Prima Donna - my favourite is Deauville range. Pretty and effective.
Interested in your idea of having something made, though haven't tried it myself.
Good hunting!