I personally would not have had any problem with Cherie wearing a trouser suit. But it’s not my house and so, inevitably, not my rules. If people I visit have strictures, I will comply (shoes off/shoes on is the commonest one I can think of)
At about the same time period as the Blair’s visit to Balmoral, I was working in a large firm which openly discouraged women from wearing trousers. So I wore skirts, because although I personally thought it was a silly rule, it was their office and their rules.
So in Cherie Blair’s position, I would have worn skirts. Unless I deliberately intended to ruffle feathers. Reading her account of her trips to Balmoral, that may well have been the case.