Keep in mind, this will be policed only on poor ethnic minorities. Rich white guys in their private club s will still smoke with impunity.
This is the real answer right here - this is just another poverty tax/punishment.
I don't smoke, never have, but I know why people smoke, and it's now (that it's no longer seen as "cool") almost exclusively to try and relieve a tiny bit of the mountain of stress that existing in the world today (especially as part of a marginalised group) brings, and there are a million better ways to reduce the need to smoke, and improve the health outcomes of smokers (eventually, hopefully, to the point where they are able to reduce smoking or stop altogether).
Sunak is looking for a quick "win" for headlines and distraction, not to actually help people live healthier better lives (E: just seen his transphobic comments, which only reinforce this point). Why target the source of the problem when you can slap a band aid on it and bask in your own glory for a couple of days before your next bit of corruption is exposed?