I live in Devon and it is here caused I believe by the granite rock. I do not know how or why. When I moved to Devon it was mentioned on the surveyors report for the house that I bought to have a Radon check. When I spoke to the Estate Agent about doing this they were very dismissive as was everyone else I asked. I got the impression that although the gas in dangerous no one ever gets effected by it. Also as it is a heavy gas if you do get a build up it will happen in low areas of the property. The advice is was given was to open your windows once in a while and if there is a build up it will get blown away.
I also had a quick look on the internet and did find this BBC news article
BBC NEWS | Health | Radon gas action needed across UK
and this which seems to explain about the gas
HPA - Radon