Google it a bit more, you will find reports of professionals that have tested their granite worktops and then promptly had them removed.
Granite produces randon, and on average a lot more so than other stone materials. The issue with radon is you breath it in, which makes the dose you get much higher, than the same exposure from other radiation sources. Some granite produces much more radon than other bits and they might have come a few meters apart from the same quarry.
There is no such thing as a safe level of any radiation - period. All extra radiation increases the risk of cancer. Install a million granite worktops in the UK and expect a small but none zero number of people dying of lung cancer down the road. Not worth the risk when there are alternatives that are just as good and don't carry an elevated risk if you ask me.
I notice the site you list does not mention Aberdeen - the granite city at all. Granite used extensively in the buildings, in the ground, biggest radon problems in the UK...
I have absolutely no axe to grind, with zero ties to any worktop manufactures or radon testing kits or anything else, just an IPEM accredited Masters in Medical Physics.
