This is Prickly Sow-thistle, Sonchus asper. It differs from its smooth cousin in having a shiny, darker green upper surface to its leaves. Also, the bases of the leaves have spectacular lobes which clasp the stem and curve downwards.
The flowers look very similar to those of Smooth Sow-thistle and the two plants can be difficult to tell apart. To really get the i.d. bulletproof you can count the vascular bundles in the stem or get the achenes (seeds) under a microscope, but I don't suppose many people would consider such lengths necessary or worthwhile.