It is a Euphorbia.
But which one...
It has some of the bluish colour of E. characias, and the pale midline stripe of E. schillingii...
I really can't be sure, even of the species, let alone cultivar/hybrid, if any.
It's Caper Spurge (Euphorbia lathyrus). It has explosive caper-like seed pods which chuck the seeds some distance. The seeds can also lie dormant for a long time waiting for soil disturbance. It is pretty fast growing but it must have been lurking a little while before you noticed it.
The latex is rather irritant so if you do damage it then be careful to wash your hands. It allegedly deters moles!
all the best