NRU74, I see what you mean. I seem to remember Dame Kitty at the Royal Albert Hall. Her dress had been adjusted with a choice of fabric that remained hidden.
Only at the crescendo of Rule Britannia was the Union Flag revealled.
As for dress shirts with a choice of backs, if tailors dislayed them at the staff college surely tailors sell them ready to wear. For a bespoke shirt there are many shirtmakers in London, many of whom advertise in Jermyn Street.