I am going from London to Paris on May 21 and returning to London on May 25-- should I book my train tickets now, March 6?
Yes. It would have been better to book them even earlier. Some trains on your travel days are down to less than 10 tickets per train. Book now.
Book ASAP. The lowest fares are offered 120 days out. The fares just keep going up as more tickets are sold and you get closer to your trip. Book on Eurostar.com. One clarification to Leslie's post. When you see "less than 10 seats" on the eurostar site, it usually means less than 10 seats left at that price. Once those seats are sold, the price for that class of ticket will jump.
Absolutely, its only going to get more expensive and actually, budget tip, by booking a round trip you might find some extra discounts available if you search for them online.
Yes book now as others have stated.
Thanks ---- I did book and the earlier departure already is more than the later return