Regarding flowers for the Royal Wedding......
Before it got lost in the move and destroyed in storage, I had a book of English Floral design authored by the lady who was in charge of the Royal Wedding of Prince Andrew and Sarah Ferguson. In that volume, it actually covered the Royal Wedding....and covered a few traditions.
If Kate follows tradition, there is one specific floral material that WILL BE in the bouquet, It is a sprig of myrtle taken from a tree planted by Queen Victoria. Also, technically, the bouquet itself is made by a member of the "Worshipful Company of Gardeners" which I take that to be a type of British Floral Organization. After thinking...I mnight have gotten the name of the organization wrong......it might be "The MASTERFUL Company of Gardeners rather than the Worshipful Company...
Regarding cost, from what was covered in the book, there is a fund called the "Privy Purse" which covers things like that. IIRC, the amount allocated from this, plus gifts of flowers from UK growers.....was WAY MORE than enough to cover the actual expense.
Some of the things to took for in the Abbey for florals to be placed on......that were decorated for Andrew's ceremony - The pulpit, the tops of the Henry and Catherine's towers, the arch leading into the North Transept, along the choir rails, on top the altar itself, There are two large stone triangles that were covered with florals.
All in all, it was a delightful book and some of the british techniques, I have put to use on more than one occasion.