To travel from Paris to London is an incredibly easy thing to do. Eurostar trains leave the French capital continuously, and by making use of the tunnel underneath the English Channel they will get you to the heart of London in no time. These days, the Eiffel Tower and the Big Ben are only 2 hours and 20 minutes apart.
You have the following train options for this route:
Eurostar: approx. 2hr 20min. Reservation mandatory
This train is not included in your Eurail Pass, but there is a special fare for pass holders. See the Eurostar website for more information.