Vaping may cost the government too much. If everyone started living an extra 10 years, that's an extra10 years of pensions, 10 extra years of healthcare, etc.
Although the TPD2 is a EU directive, I doubt that a Brexit would lighten the load in terms of regulation. The UK (via its regulatory vehicle, the MHRA) still clearly want to appease public health concerns they feel are lacking under the currently provided GPSD. Although not a direct transposition, it would probably feel like a close 'cut and paste' job that keeps in the spirit of the directive. IMHO