There are two common species of sow-thistle (Sonchus species) that are very similar and I am not competent enough to tell them apart and so I combine my sightings under the name 'common sow-thistle'. The two species are prickly but also known as rough sow-thistle (Sonchus asper) and smooth sow-thistle (Sonchus oleraceus).
They are different, of course, but the defining factor is whether the auricles of the leaves are rounded or not! Rough sow-thistle can have brighter yellow flowers, may have more pointed leaves, may prefer lime soils and is less likely to be seen in winter months. Smooth sow-thistle is probably the nore frequent of the two. Overall, they are quite unmistakably thistles but with yellow flowers that occur in clusters, have prickly leaves and exude a milky substrance if broken.
Undoubtedly weeds of cultivated areas the seed and spread quickly. The leaves are edible and are popular in Chinese cooking and the plant are used by herablists as a cure for gall stones, liver and kidney complaints and piles.