These are indeed the flowers of the salad leaf, Rocket.The flowers are edible and look very pretty on salads. Bees and other insects love the flowers and they usually set seed easily, giving me a good supply of seeds for next year's salad crop.
I have been collecting lots of seeds from flowers whilst the weather has been dry. When the seed is properly dry, I clean it up and send it to The Cottage Garden Seed Bank where it will be added to the catalogue for this year and members select the seeds that we would like!
In fact, the Rocket in this blip grew from seed donated last year.
ADDED LATER: if you like the idea of Edible Flowers this site has lots of suggestions!