The smoking ban is incredibly popular and supported by most people across all generations, also given we have tax payer funded healthcare this is a clear win for the nation.
That's about to be turned on its head. Banning smoking will make it seem edgy, transgressive, and alluring. Still worse, banning it only for people below a certain age will make it seem mature and sophisticated to those below the cutoff. The forbidden fruit.
The UK Govt couldn't possibly devise a better way to popularize cigarettes with young people. And they'll create a nice little black market, to boot.
or to put it another way --- the phenomenon you point to is real but completely swamped by the more direct impacts
In the UK, however, the way to bet is that the government enforces the ban with maniacal zeal, so smoking rates will probably go down slightly in the near term. (For better or worse -- usually worse -- the UK has more state capacity to enforce a ban than the US or indeed the vast majority of other countries.) But smoking will become, once again, a prestigious activity. All the cool kids will smoke, if only for the social signaling benefits. Over more than a decade, youth smoking rates might surpass what they were before the ban.