Charlock and Wild Rapeseed plants are common on land which may have been used for tillage in the past, or if Oilseed Rape has been grown nearby. These seeds can remain dormant in the soil for up to 6 decades and germinate when the soil is disturbed for growing wildflower seed.
You should cut your wildflowers back when the plants have finished flowering and the flower head has dried out. This is usually in September. Remove the cuttings from the area.