Arriving in London by rail
Major train stations in London
(If you count them all up, there are actually 366 passenger rail stations in Greater London.)
Arriving from the airport
Arriving on the Eurostar
The travel time between London and Paris (Paris station: Gare du Nord) is actually about 2.5 hours (it varies from 2:16 to 2:37)—though remember that the U.K. is one hour behind central Europe, so once you factor in the time change the trip to London will take about an hour and a half (at least according to the clocks), a trip from London to the Continent will arrive 3.5 hours later on the clock.
London to Brussels-Midi/Zuid is about 2 hours; London to Amsterdam takes about 4.5 hours.
You are required to check in at least 30 minutes before departure (10 min. if you overpaid for Business Class), and they airport-style secutiry lines, so plan on getting to the station well ahead of time—I'd say at least one hour ahead of your scheduled departure, just to be safe.
What if I took the ferry?
Trains coming from Dover (where ferries from the Continent land) arrive at either Victoria Station or Charing Cross Station, both in the center of town (10.5 hours total travel time from Paris via the ferry route).
Arriving from Edinburgh
If you're coming from Edinburgh (or Hogwarts), you arrive at King's Cross Station in the northern part of London.