You really do not need to turn up so early at Eurostar, and a lot of the reason for the overcrowding is because people do so. At very busy times, they do stop people coming in too early and actively encourage people to leave the queue and come back later - but still people like to queue through the station shopping area, via the circle, out the door and then along the main road. Then they moan, and the media come to film the queues, about how long it takes to get into departures.
Eurostar isn't an airport. You don't need to arrive two or three hours early. The obvious caveat is that you want to make sure that if you're coming from a long way away, you get to the general area in plenty of time - but go enjoy some food or a drink elsewhere in the station, or the station surrounds (go have a wander around Coal Drops Yard or something, depending on how much luggage you brought). If you've had a drink and gone to the toilet outside, you want to spend as little time 'airside' as possible.
Business Premier customers can go into departures whenever they want, but even then there's not much reason to be there any longer than necessary IMO.