I think the Queen will choose to wear brooches that have Commonwealth association, to symbolise her position as not only Queen of the United Kingdom, but also Queen of the other Commonwealth Realms as well.
As such, I think we'll definitely see the
Wattle Brooch (a gift from Australia) and the
Fern Brooch (a gift from New Zealand). "Safe" choices such as the
Flower Basket brooch, one of
Queen Victoria's Bow brooches, Lover's Knot brooch, one of the
Emerald and Diamond brooches will also probably be seen.
What we will almost certainly
not see at any official function are any of the Cullinans.
And while we are on the topic of Brooches, attached is the Cambridge Pearl Pendant the Queen wore for today's meeting with the Prime Minister.
Pearls and diamonds - what can be better? Apart from diamonds-only, diamonds and sapphire, and diamonds and emeralds, that is.