Two self-seeding biennials: Garlic Mustard (Alliaria petiolata) and Honesty (Lunaria annua).

 Both are of the Brassicacacae family of plants i.e the mustard/crucifers/cabbage family. Both are important food plants for butterflies -and not just the flowers. Several species of butterfly (e.g. Orange Tip) lay their eggs on the buds; later the caterpillars feed on the seed pods.

 Garlic Mustard is native to the UK and prolific. Honesty has naturalised and is fairly widespread. They pop up all over the place in semi-sunny/semi-shady spots. In fact A. petiolata has another common name which accurately describes its main habitat: "Jack by the Hedge".