I don't know what it's like over there (either cigarettes or e-) but here I would say anecdotally that a lot more school-age 'young people' smoke cigarettes than just-after-school (i.e. university students & early career); so perhaps 'those [young people] who already smoke' is quite a different group here? It'd be interesting to see the underlying not-anecdotal data, for sure.

To clarify - I mean that perhaps British young people that would ever experiment e-cigarettes are already experimenting with cigarettes, whereas American young people are less inclined to (I think the legal age is mostly higher, for a start?) and so the intersection with e-cigarette users has a smaller, and different, starting point.