I'm going to chime in here. One of the reason to wear 'British' is to expose British brands, and also to hopefully generate some income in hopes that the public will shop at these brands. I've said this before, but I think it's worth restating. There is no way in he.. the normal person is going out to purchase every high profile outfit any members of BRF wears, especially if it costs more than 1,000 Pounds. What sells the most are the accessories, the items that are more readily available (shoes, handbags, sweaters, etc.). I don't know any 'normal' person, the majority of the public who would go out and buy an Erdem, an Expensive Burberry dress or suit, McQueen or even Givenchy just because any member of the BRF wears it.
There is this expectation that if the DoS were to all of sudden begin 'wearing' British brands that revenues would increase, which is not always the case. I also put 'wear' in quotes because let's face it, she does wear British, however there are some who think that her accessories don't count. What sells the most are her coats, jewellery, handbags and shoes. And these are precisely the items where she wears British. That's where the revenue is generating from.
Personally, from my perspective, if the DoS was to all of sudden be seen in majority British brands, I cannot imagine what would be said of her and I can just imagine the comments. Right now, she is doing a mixture a both, and I don't expect to see a swift change from that to mostly British. I think that we will continue to see a mixture of Commonwealth, British, American and of course her trusted Clare Waight Keller for the foreseeable future. And that is alright with me.