Key word is essentially.... Basically if you get on in London you can get a feel for how full the cabin is... There are only 2 possible stops where people get on when you depart London (and no where near all trains stop at either), Ebbsfleet and Ashford for which you can take a chance leaving London whether people will get on at the other 2 stops (worst case, you move back to your original seat). Lille is a red herring leaving London as nobody gets on there when heading for Brussels, people only leave. Coming the other way treat Brussels as for Ebbsfleet nd Ashford, ie if someone gets on and is assigned the seat you over to, worst case scenario is your original seat!