I do not have a clue what it is, but it is large (very, - a good 6' tall with 4' spread) with thick (mallow-like) roots, large glossy toothed and grooved leaves, and some simply amazing flower spikes with flowers that seem almost orchid-like (but surely not, - no orchid that I am aware of grows to that size in the UK, - outside of Kew Gardens, anyway).
As far as I can tell it likes damp locations and is currently growing on poor soil with a mix of rocks, and rubble, etc. close to a main road.
This is Acanthus mollis.
According to Wikipedia, it's native to the Mediterranean region, "from Portugal and northwest Africa east to Croatia and it is one of the earliest cultivated species."
I can't imagine why it would be one of the earliest cultivated species (except that it is so easy to "cultivate"); it isn't pretty (my opinion), and you don't eat it. Also be careful ripping it out, its flowers have some nasty spikes that you don't really expect.
I don't know how common it is in the UK, but we have a lot of it in my area (Western USA) mostly because it is a pretty common landscape plant. And we know the classical Mediterranean cultures grew it, because, to quote Wikipedia again, "The shape of the Acanthus leaf ... inspired the ancient Greek sculptor Callimachus (5th c. BCE) to model the capital (crown) of the Corinthian column. Since then, the Corinthian order column has been used extensively in Greco-Roman architecture. Virgil describes Helen of Troy as wearing a dress embroidered with acanthus leaves."
