Maybe an aster
Near Ecomuseu da Marinha da Troncalhada, Aveiro, Portugal.
Found along a canal in London. I don't know British wildflowers at all; can anyone help? ~4 cm across. I have a growth-habit shot I can upload if it will help. The petals curl as they age; that and the very long leaves are fairly distinctive.
These asters are blossoming and being visited by myriad honey bees. They self sow. Last year I was told to cut them before flowering due to fungus. Normally I would cut the dead stalks during the winter rains, stripping the leaves and using the stalks for art projects. This year the fungus was on the leaves, but no one told me to cut them down and they are fine- some mildew on lower leaves but otherwise they look healthy. The patch has been in place for several years and appears to move slightly southwest as time moves on.