This looks rather like Allium sphaerocephalon, a European native, that is growing like an invasive garden escapee on your Cornish cliffs.

I agree with Bulbaholic about it actually being a garden allium that has gone feral but, apart from the fact that it is clearly an allium I wouldn't like to say what type. There are dozens, or maybe even hundreds of them. I've got about 20 different ones which all self seed happily (eventually).

The thing that surprised me about them is that I've never seen them before and they appeared in three different places - twice in hedgerows & once on a clifftop. And the places were a good fifty miles apart.