In the absence of a real FAQ, I assume their BGA is similar to FTDNA's and Dr McDonald's. If so, draw a line between the British Isles and the Caucasus (the middle of the region highlighted for "Russian/Persian/Turkish" in this blog post: http://www.yourgeneticgenealogist.com/2012/03/ancestrycoms-a...). Then pick the spot 6% of the length from the British Isles, which would place your genetic average somewhere around Luxembourg.

Otherwise, your results may simply be similar to British Isles based on the underlying Celtic and Germanic ancestry in both places. Or the particular mix of your DNA could be confounding the algorithm to come up with a better fit.