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

Granite tends to contain Radon as it's in the decay chain of Uranium, which is often found in Granite.

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.

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

The map shows Reading does have some radon. Probably not at such a level most people do anything about it, but for a basement you'd definitely be better safe than sorry.

Very much so.