Not only the population of UK has increased in this time, it has also gotten older on average. Moreover is 0.002% of population getting GBM per year even significant enough to blame mobile phones (that is used by everyone)?
Different ancestries carry with them different genetic predispositions. It might be that the GBM rise is due to phenotypes not phones.
[1] http://www.migrationobservatory.ox.ac.uk/resources/briefings...
Though I guess if car crashes were a common cause of death that doesn't require autopsy, you'd see some difference in GBM rates between car heavy places like the US, vs places with better public transit like EU.
Every study that measures disease or death rates takes this into account. They use standard populations (standard millions) to create age-adjusted statistics. In this case it was European Standard Population (ESP–2013) they used.
The article reports increase in all age groups. The article is not making any link between brain cancer and mobile phones. There is just little speculation to crate headlines.
Then you can start adding in the things that make people live longer, like less heart disease per year of life, etc., and you get the same effect they are charting.