The Goldsmiths and Silversmiths Company Ltd was founded in 1898 and lasted until 1952 when it was absorbed into Garrards, the Royal Jewellers. During it's life it states that it was By Appointment to:-
His Majesty The King.
Their Royal Highnesses the Prince and Princess of Wales.
H. M. The King of Hellenes.
H. I. H. The Shah of Persia
H. R. H. The Crown Prince of Siam
Her Late Majesty Queen Victoria
It made or engaged others to make all sorts of things, swords, watches, clocks and all manner of expensive or exclusive jewellry items. Regarding your brooch, my own view is that the crown is a depiction of the British King's Crown. If so, it ranges through Edward VII, George V and George VI. It seems a very personal thing to set an enamelled monogram attached to and below such a prominent crown, perhaps indicating a member of the family. Rather more abstract, it could be some association of which a King was patron. I've drawn a blank, but my resource is very limited. If nobody comes along with an idea, I suggest that you send your photograph's to the Jewellry Department at Sotheby's London to see if they can help. I have had such dealings with them and have always found them nice and helpful and very knowledgeable.