I'm landing at LHR and need to go to Paris. Is taking the Tube to St. Pancras and then the Eurostar the best way to do it?
Not for me it isn't. BA goes from LHR to CDG a gazillion times a day for ninety bucks. The flight takes an hour.
If you were to spend a night in London, then the Eurostar wins hands down. Since you want to go directly to Paris, Ed has given you the best option.
Hate to ask a dump question but why not just fly into Paris? Unless you are spending a few days in London.