See http://seat61.com/France.htm#Eurostar

I would not spend most of the day traveling for a few hours in Paris. Only you can decide if 6+ hours of travel in one day is worthwhile. I would find it exhausting.

Of course it's doable.

It's a long, exhausting day, but it's definitely doable and is actually one of the cheaper ways of taking the Eurostar. It leaves from St. Pancras in London and arrives at Gare du Nord in Paris. But the cheapest way to do this is to book a nonrefundable ticket far in advance.

However, if you have never visited other destinations in England, perhaps it might be better to take day-trips from London to other interesting places like Oxford, Stratford, Cambridge, or Bath, all of which are easily done as day-trips from London.

I love the ease and convenience of taking the Eurostar from London to Paris but I wouldn't do it for one day when there are so many great places to see near London. You might go to Bath in the south of England, a 90 minute train ride from London's Paddington Station. Bath is small and lovely and easily walkable vs Paris which is vast and can be confusing. You can cover quite a bit of Bath in one day vs Paris where you won't scratch the surface.
Ann Marie

If doing it ask for a Day Return ticket - there is a special fare for these - two hours each way and you're in the heart of Paris - having all day there - www.eurostar.com for fares and yes like Doug says above booking way in advance gets the cheapest fare.

Depending on which train you take the journey is 2.25 to 2.5 hours. Plus the 20 minutes or so that you'll need to arrive before the train to check in. And the time to get from your hotel to the train station plus the time on the Paris end from the train station to whatever sights you plan to see. So it's much more than a 2 hour trip.

Eurostar says to check in for your trip at least 30 minutes prior to departure. Not knowing exactly how long the process would take, I got there about 50 minutes prior to departure.
Ann Marie