Under 17s aren't being banned from buying cigarettes when they get to 18. The legal age will rise to 19 on 01/01/2027. Then it will rise to 20 on 01/01/2028, then 21 on 01/01/2029, etc. It effectively bans those born on or after 01/01/2009 from buying them, but only because they aren't (and never will be) old enough at any given point.
The cigarette industry isn't a fair game. We know that cigarettes are highly addictive. We know that nicotine reaches the brain within 10-20 seconds of smoke inhalation, and we know that addiction can be formed within days. The only way to stop it is to literally stop people ever using it.
Some people will still acquire cigarettes but that isn't a reason not to try and save a generation. There are literally no benefits to smoking that can't be achieved by healthier, cheaper, means.
Soon it will be obvious if someone has illegally acquired cigarettes because of their age. That will reduce (not stop) the number of smokers in public. Other things will replace them but we will see a shift in behaviour.
Single use vapes are already illegal and the new bill is going to bring in restrictions on packaging and flavours of vapes. People shouldn't be expecting tutti-frutti and getting lung disease.