The train in particular is my choice. However, you will not be able to book any earlier than 90 days out because the train times are not in the system. Early booking is better typically because you will get better fares.
There is a direct TGV into Gare Lyon in Paris - trip is approx. 3 1/2 hours with multiple departures per day. Look for the Piccolo fares - they are typically the least expensive - but the most restrictive (no changes). Reservations are required on TGV trains. (off season tickets are about 53 Euros per person one way with advanced booking)
www.sncf-voyages.com
scroll to the bottom and click the British flag to change to Engish.