Used to be a much greater proportion than it is today, largely thanks to the growth of places like the Amsterdam Internet Exchange.
At one point there was a building on the Isle of Dogs (former island in the Thames) through which an absolutely terrifying proportion of the network traffic for western Europe travelled.
At least as far as visible hops in a traceroute go (admittedly doesn't cover everything, such as fiber-level switching), most of my traffic to the U.S. from Copenhagen currently seems to bypass the UK. A few common transatlantic endpoint pairs seem to be Paris-Ashburn (he.net), Amsterdam-DC (hwng.net), and Copenhagen-NYC (tdc.net).
