Just did it a couple of months ago. You can do it all online or walk over to the concierge desk when you get in and tell him your itinerary requested. He booked it all for me, told me what time to be in the lobby, had a cab waiting( it was raining it's an easy walk if not) and told me what stand my train would be at. I had gone to dinner and a few cold ones with a few of the boys in purple. I threw him a healthy euro bill. Well worth the money! I didn't even break stride to step on the train and was on my way. Definitely plan for some time for customs and realize it is the silly season right now with all the tourists. One or 2 first class tickets shouldn't be a problem. The change in London is really easy! just did it in reverse with my wife and ankle biters 2 weeks ago. cost me 27 euro from London to Cambridge. Highly recommend the train over the flight!