The law states:
“smoking” refers to smoking tobacco or anything which contains tobacco, or smoking any other substance, and . . .
smoking includes being in possession of lit tobacco or of anything lit which contains tobacco, or being in possession of any other lit substance in a form in which it could be smoked
[Health Act 2006, Section 1]
eCigs aren't covered by the legislation but many pubs and restaurants have followed the lead taken by J D Wetherspoon in banning them and it's likely that most supermarkets would also ask people to desist from using them.