There's a direct bus from Waterloo to St Pancras:
Take a train from Farnborough to Waterloo.
Leave Waterloo station by the main entrance (which is off to your left as you arrive on the station concourse).
Follow the main flow of people heading to the left.
After just a few yards you'll come to a zebra crossing. Cross over it and walk through the archway that passes under the tunnel. (Picture here:
http://i60.tinypic.com/160ao7m.jpg ).
That takes you to Bus Stop K, on Tenison Way.
Take a 59 bus (very frequent) to St Pancras International. (30 minute journey).