Originally Posted by
teflon
though I'm not sure I'd risk anything less than 3 hours between landing at Heathrow and leaving St Pancras)
I can't see the point of going from LHR to St Pancras to catch a train at all.
That 3 hours consists of 1 hour immigration / collect luggage / customs, 1 hour getting to St Pancras / 30 minutes check-in time leaving 30 minutes for "unexpected".
Contrast that with the same 3 hours getting another flight
1 hour immigration / collect luggage / customs, 20 minutes changing terminals / 40 minutes check-in time leaving an hour for "unexpected".
And that's assuming that you can't get your luggage checked through to your final destination in which case you end up with 20 minutes changing terminals / 40 minutes check-in time leaving 2 hours for "unexpected".