Getting into London Heathrow late Friday night (11:00 pm - into London by midnight or so)), then first train to Paris on Saturday morning (6:55 am) - St. Pancras opens at 5:00 am. Would prefer to avoid getting a hotel (my brain will still be on Eastern Daylight Time) - any thoughts on what to do for 5 or so hours? Was thinking an all-night or late night bar, preferably around St Pancras.
If your flight arrives at 11, it will be close to midnight when you exit customs/baggage. Instead of taking the train why not take the N9 Night Bus which starts up about midnight and runs about every 15 minutes. Its a local red public bus (double deck) and will take 90 minutes through London's suburbs. Its also the cheapest way of getting into London from Heathrow.
Terminates at Piccadilly Circus, which is the heart of the nightlife in London. St Pancras is a little out, if you find a late night bar or drinking place away from the station its the kind of neighbourhood where you might not be safe to do so.