" I thought that, in this country (UK) at least, church attendance has been falling, consistently, for decades?"

Just because persons do not belong to, or even attend, a church, it does not automatically put them in the "Atheist" camp. It may merely mean that some, or even many, no longer have a need to follow an organized religion, nor visit a church to satisfy their "God believing" needs. There are more accurate avenues to determine belief statistics than this single one.