On the London-Paris route there is standard class and first class. As the trip is only 3 hours, it's not a great difference; 1st has a little more room and one side of the aisle is in single seats, the other double.
Standard seats are 2-2, little less leg room, and tend to be more crowded.
If the fare difference is not great, I would go with 1st, otherwise standard is ok for a relatively short journey.