Ammi Majus (Bishop's flower) has lacy, white flowers, like a more delicate form of cow parsley. Ammi Majus is the best white filler-foliage plant you can grow and spectacular arranged in a great cloud on its own. It forms larger, more prolific plants from an autumn sowing.
Centaurea cyanus 'Blue Boy' - Cornflower - can be bought in bunches of 50 stems - cost around £30 max. I could put you in touch with some British flower growers - you may be able to negotiate prices yourself and want to use British grown?