UKradon, a British company that sells "Radon Risk Reports", has created a helpful and eye-opening map of England and Wales (Sorry, Scotland!) that shows which areas in the two countries are most afflicted by dangerous levels of radon gas.
You can see the UK radon map here.
In general, the Western parts of the Isle are much more prone to radon than the Eastern parts, which are relatively free of radon risk, according to the map. North and South appear relatively equal.
Overall, Southeast England has the most dangerous levels of radon gas: over 30% of homes in this area have radon problems!
But, if you live anywhere in England or Wales, I suggest you take a look at the map for yourself.
Update: we now have a radon map for Northern Ireland, too!
Radon Gas Map: UK