I would be curious to see a poll in Britain and in mainland Europe as to whether Christians or muslims are the greater threat.
Given that a British conservative here on FR had said from their viewpoint, the American conservatives look only a minute degree to the left of Genghis Khan, and various leftist and conservative papers in Britain both report British as a whole has high suspicion of America's "serious Christianity", I would imagine they would view real Christianity as a greater threat than Islam.
This would be even more so in Europe - France is the ultimate ACLU country.
I believe Muslims would easily win that race. Of course were you to include the USA as a choice for who is the greater threat I'm sure that we would win hands down.