Slender Thistle (Carduus tenuiflorus)
This is an uncommon species in Britain and is found mainly near the coast, especially on cliffs in the south of England. There is a lot of it on the cliff tops near St Adhelms Head and it also occurs on the Purbeck Ridge at Ridgeway Hill. I expect it does occur elsewhere but those are the only two places I have seen it so far.
It flowers from June to August and is at its 'best' about now although this is a plant that is never really at its best, it is a bit of a nondescript thistle which not much going for it!