Not sure we have the Eastern tent caterpillar here, but we do have other tent caterpillars, and the effects are the same; they build a tent full of caterpillars over an area of a plant and eat everything within it. Probably Ermine moth tent caterpillars are more common here and in Europe, and they can cover metres of hedges, defoliating them.

the defoliation doesn't actually kill the plants as they send out new leaves once the caterpillars move on. so whilst they can defoliate their food plant and it looks unsightly they provide food for other wildlife.
I don't think we have that species yet in the uk but as bamboo says we have our own native tent moths.

Thanks all! it was the first time I'd seen this and remembered reading about it over a couple of years ago, from some one that gone hyper on it, so held in mind the caterpillar coloration. Looked it up on the internet and came up with Tent Moth in America and assumed Eastern. Next time I'll go to the doctors to see if I have the flue or the plague.