Apparently there are competing explanations, it could be about Britons looking roasted when they try to tan on French beaches. 'Lobster' is another name for that, and we do use it mostly in relation to people from the UK (amicably of course)
Roobarb and James, is this link working for you? https://www.youtube.com/watch?v=GZvrcjDWc70 (it might not, but if it does, this is the best Frenchglish love song I know, and it contains the famed word 'rosbif'.)
Yes I live about 80km from the Mont Blanc nearer to Geneva. I'm originally from the South, so I'm more of a sea kind of person. Lake Geneva is big enough I guess...