The reason why they're reducing pack size is because the tax on tobacco went up in the recent budget so instead of pushing the price into say £7.50 or whatever it would be. Mind price varies between location and shop, they decided to take 1 out last year when again the budget announced a rise tobacco tax so they took another one out. If a Brand like sterling which where I am in Scotland costs almost £7 for a pack, without removing two it could be £7.50 or whatever which pushes people into a new psychological barrier of cost. Brand loyalty is gone in the tobacco market so people are going cheaper so they want to keep the price down and unfortunately since most of the price is tax in the UK they can't exactly cut into their own profits and they can't deny the government tax so they're forced to chose what to do.
Unfortunately with tobacco products being hidden now and no advertising on tobacco products other than online, you can't exactly blame anyone for no warning, only perhaps the checkout operator who failed to correct you when you asked for 19 or 20 pack of sterling menthol.
By the way I'm not affiliated with the company but I'm just using logic of what I've read online.