Honestly don't think a sizable number of potential tourists will avoid coming to Britain because they can't buy any tobacco here. They'll just bring what they need if they are even aware if the ban.
I also have doubts whether the law will stand for decades. NZ withdraw theirs pretty quickly.
There was a fair amount of resistance in some quarters to the smoking ban in public places but I can't see that legislation ever being undone. It's been an absolute godsend for those of us who've never smoked. My Mum always used to complain that I stunk like an old ashtray when I got home from the match or the pub