Ok, so an area that is a high radon area is an area where 10% of houses are estimated to have levels greater than 200Bq cubic metre. Problem is for a given area, that's more that 10% or 20%. It is possible to put radon barriers down - but from what I've read they don't always work.
You can get it measured....but it takes 3 months. If I was concerned I'd get it measured at a nearby house - if you know someone that is. But the down sides of that, it's a measure of their house and not the house you're going to be purchasing. - barrier differences, localised differences. But it would at least give you a reference point.