If you're planning to travel from London to Paris (or vice versa) then I can highly recommend the Eurostar train service.
We picked the train up from Ashford in Kent, the check-in was efficient, the staff courteous and the train was on time.
The train owes more to aircraft technology than most railways, the seats, buffet and toilets were clean and comfortable. We whizzed through the channel tunnel in under thirty minutes, and the whole journey from Ashford took just two hours to the Gare du Nord in central Paris.
No airport delays, no baggage carousels, no problems.