There is one daytime flight (American Airlines) to London from O'Hare. We always try to take that because it gets you to London around 10:30PM. By the time we get to a Heathrow hotel and check in, it's 11:30PM or so and we're tired enough to surf/email a bit and then crash. Wake up the next morning as usual and not have to deal with jet lag. You might check to see if Logan has a daytime flight to London (assuming you're going there first).