Since the 40D is a 1.6x body, and most everything in Europe is up close & personal, I would go with the recommendation of the 10-22, and take a 24-105, if you have it. Since weight is not a factor, why only request help on selecting One Lens?
Depending on where you are headed, if you are going to Germany/ Austria/ Mtn Region, I would take the 70-200, as an UWA won't do you much good up in the Mtns. I personally would take 4 lens selection, consisting of an UWA (16-35/10-22), either a prime of the 24/35 1.4 variety, the 70-200, & a Tilt Shift, but that is me. Going to Europe, which parts matter not, the reason, why I am suggesting the Tilt Shift, is that, I would want to capture the Architecture, but do it right, and not deal with it later in PS, which doesn't do as well as a T-S does.
Most importantly, enjoy your trip. Take in the Culture & History.