Every year New Covent Garden run a campaign to promote British Growers and indeed British florists who then go on to showcase their talent. It is worth a visit to the web site just to look at the stunning creations.
There are a few local growers I visit during the Summer months who can provide the most lovely sweet peas, dahlias, and peonies.. Also some fabulous roses can be found too. It is worth seeking them out.
Enjoy your flowers in the garden.