In granite bedrock areas such as Cornwall, yes, radon is a significant concern that requires measurement and usually remediation (extraction fans).

The chalk bedrock in Reading isn't generally susceptible to radon.

UK radon mapping data: https://www.ukradon.org/information/ukmaps

It is in some areas with a lot of granite, such as Cornwall. I'm not aware it is a factor in Reading.

It comes out of the ground. UK has some of that I think.

Tunnels and basements just trap it so it can't blow away.

Very much so.