You can get from London to Paris in a little more than two hours. It's almost too fast! The gentle cradling sensation, the blur of northern France's orderly countryside racing past, the breakfast tray of croissant, yogurt, and coffee--and the knowledge that you'll soon be in another great metropolis...it's one of the most wonderful feelings in travel, and I'd happily sign up for four or five hours of it. Why passenger flights from Paris to London even exist, I have no idea.
Note: There are several ways to book. I got this and my other Euro train tickets on raileurope.com.