I saw a post about this and now I can't find it. Is this do-able (by train) for 2 very energetic 20 year olds who will be staying in Paris but are dying to dash about London if even only for a day?

We're doing the exact reverse - a day trip to Paris. If you limit yourself to just a few sites and use Eurostar for city to city commute, I think it is doable. But you'll only see a little

Try the Big Bus trip in London, it will take about 3 1/2 hours, but you'll see the big tourist sites. http://www.bigbus.co.uk

Unless you just want to put a "check in the box" saying you've been to London, I can't imagine wanting to leave Paris and travel to London just for a "day" trip. Lonton is worth 3-4 days to do it justice.

It's definitely do-able: Londoners and Parisians do it all the time.
In fact, that's your problem. Single-day round-trip fares can be quite high. They're a great deal cheaper on Saturdays and Sundays than on weekdays. In fact, if you're forced to do this trip on a weekday, you'd probably be better off financially to fly (try www.skyscanner.net or www.applefares.net for low-cost options and www.flybudget.com for advice on how to work the budget-flight system in Europe). Again, Brits and Frenchies do it all the time

We took the Eurostar last month (from London to Paris, but we spent several days in Paris and it was not our first time to either city). My feelings about the train are based on the fact that both our trains ran late .. I could not help thinking about anyone who wanted a day trip because when trains run late your day is shortened quite a bit. You would need to take the very first train out and the last one back to make this worth while (and I still question whether a day in London would be worthwhile .. I've been dozens of times and still find new things to do/see). (One of the immigration people told us that trains run late more often then they like to say).

Fares are NOT cheap on Eurostar, and the less expensive fares require a weekend stayover. Only you can determine if this is really worth it to you.

Try Paris Vision tours (don't know if they are on the web or not). We booked a London day trip thru them and it was really fantastic. We also found it to be quite affordable.

We met up at Gare du Nord, boarded Eurostar, got tickets to take the buses anywhere we wanted in London for the day (on our own) and then met back up at the train station after. We felt the trip was really worth it, even if it wasn't quite long enough.

Good luck.