We've done London to Paris (two years ago) and Paris to London (in May).
I suggest getting to St. Pancros at least an hour and a half early. Be aware that they don't let you into the platform area and through customs more than an hour or so departure. The station is well worth the visit.
How much luggage will you be taking? Where will you be staying in Paris? The hardest part of the London to Paris trip is getting a taxi. Gare du Nord is in the heart of the city and there's little room for taxis. You and everyone else on the train will be trying to get one. They can only allow two or three taxis at a time at the taxi station. It took us 45 minutes in the queue. Plan accordingly. If you don't have much luggage and are familiar with the Metro (or just adventurous), you can use it to get to your location.