An annual flax with red flowers that are usually larger than cultivated blue flax; linear to ovate/lanceolate leaves, borne alternately.   According to Stace (4th edition), blue cultivars exist.

Blue-flowered cultivars require careful examination to distinguish from other species of flax

red flowers

Photograph whole plant in habitat

parks, gardens, waste ground; may occur in annual 'wild-flower' seed-mixes and persist for a while. 

non-native, casual