Current data on Background Radioactivity in the UK
I've seen different textbooks list sources of background radioactivity in various different ways, sometimes quite vaguely or even contradicting each other, to the extent where wasn't sure whose data I should use to teach from.
Finding this a little irritating, I went digging and found that Public Health England (and its predecessor agencies) have been publishing summaries of their research into this situation for quite a while. The latest report (Oatway et al., 2016), based on data for 2010, makes for interesting reading, particularly when they make it clear which quantities they're estimating and which are based on more accurate measurements.
The previous report (Watson et al. (2005)) reported largely similar measurements, as you'd probably expect, but there are some interesting differences if you want to get into it.
I've put the data from both reports into Excel, including a tab that presents the latest data in a simple table suitable for school use:
The Institute of Physics produced a lovely clear worksheet which allows you to calculate your own dose, based on where you live, how much you fly and so on.
There's also the extremely fine-grained measurements of Radon levels across the UK - my classes enjoy zooming in on certain local areas and making scurrilous comments about where their friends live.