I've never seen a herbaceous plant so tall - not even banana plants when working abroad. The biggest ones were probably about 15 ft high, perhaps more!
They look like a kind of teazel, but teazles are supposed to be about 4-5 ft. The last two pics are of the only ones left after they were cut down as they were taking over the car park! These are small compared to the ones photographed earlier ( a couple of months ago), but still seemed well over 7 ft high.
Like a common teazel the whole plant was very spiny, including the leaves.
But they are, as you point out, enormous. I find references of heights upto 3m, but not 5m!
I have to ask how sure you are about that height estimate...
If they really are that tall, I would suspect they might be growing in nutrient enriched soil or water (perhaps a sewage outfall, or a field drain that happens to be providing fertiliser due to excess application). I suggest you contact the Botanical Society of Britain and Ireland; you could point them to this discussion if you like. (Let us know.)