Anyway, found this nice write up [1] on the history of the town.
[1] http://allaboutbeer.com/article/burton-on-trent%E2%80%94the-...
Background radiation is about 15 cpm on average, and right next to the bottle was in the same range.
Doesn't appear that there is any danger.
You could experiment with pouring some salt into something carbonated. I believe it would bubble more vigorously than before.