The pre-war Ceremonial is similar-ish to the current Bandmasters uniform - most famously - as has been stated - 'twas worn by HRH Duke of York (later George VI) when he married Lady Elizabeth Bowes-Lyon (much later Queen Mum)
There is an interesting precedent for a new or "one off" ceremonial uniform. IIRC, when Capt Mark Phillips married Prinny Annie, his recently amalgamated fractional cavalry (QDG??) hadn't got around to designing a new ceremonial uniform for the new Regt. One was speedily designed, and the first two ever were worn by Capt Phillips and his Best Man (whom memory insists was called Eric - which seems a rather un-cavalry name...)
Don't know if it was then kept for the Regt, which has doubtless amalgamated again .....