They've been here in the UK since 1888....accidently brought in with a consignment of oysters from the US [probably into Southampton].....and now smother much of the South and SE coast.
I don't know when they moved around Landsend....they weren't in South Wales when I was there in the 60's....and I first came across them in the early 70's when I was a student and used to fish beaches in Sussex. They were a good bait [or bait addition] then, and still are....esp after a blow.
The points above, are all an attempt to slow their progress into new habitats......whatever we do here will have no consequence....just don't take them on a trip !!