Tbh I wonder about the premise of basing figures for lung cancer mainly on smoking in the first place. Some people smoke most of their adult life without developing lung cancer... some people who've never smoked do develop lung cancer.
There are perhaps several other changes that could have had an impact on lung cancer rates over time. When I was a bairn in Sheffield, a lot of old buildings were covered in grime and gunk due to all the pollution kicked out in to the atmosphere in the city's industrial heyday. As heavy industry declined and new regulations relating to emissions came in, the air quality (at least in terms of visible deposits) got better. This may well have had an effect.
Likewise, there was a time when a lot more people heated their homes with solid fuel, open fires and the transition away from that towards either enclosed solid fuel systems and gas may have had an effect.
The same could be said of the move from leaded to unleaded petrol.
The cause for movements in rates of cancer could well be quite varied but whatever the causes, the knock on effects of eliminating/minimising them would take a good few years to filter through.