All this really does is illustrate the variation in selenium content in the top soil in the UK.
Even if you grow most of your own produce, the actual amount in the produce from your patch could well be higher or lower than your neighbour.
Adding the image to a reply so that it can be expanded much larger!
This map (and similar maps for many other elements) is available from the British Geological Survey:
UK: Topsoil geochemistry
bgs.ac.uk/datasets/uk-topso...
A shame Scotland, in particular, isn't better covered.
Interesting that the west has much higher levels than the east. However, so much of our food is imported now, it probably is generally irrelevant.
Though when you go to the shops, a lot of brussels sprouts, potatoes, carrots, turnips, mushrooms, are grown in the UK. As well as much meat. Though that is complicated by not, in general, knowing where their feed comes from.
If you look at potatoes in particular, a lot are from Lincolnshire and Norfolk - which appear low. But some are Welsh which are likely to be high. So there are circumstances in which it might be possible to influence intake fairly directly. However, the Marches are low. Which means you might have to look ever closer! 
Very interesting helvella; thanks for sharing 👍
The really dark areas with the highest concentration of selenium in the soil are National Parks - Lake District, Snowdonia, Brecon Beacons. These are areas where most of the farming is livestock. The other darker areas are hilly regions, ie the Pennines, Yorkshire Dales, where again most farming is livestock.
Just goes to show how arable farming depletes nutrients from soil and why we have to fertilise fields. (Or take supplements)
Selenium requirements
Selenium
Naturethroid in the UK?
Selenium
