Heathrow to St Pancras/Kings Cross
 

I land in Heathrow on Friday Nov 16 at 6am. I am trying to book a train to Brussels there is one at 8:05am and one at 10am from St. Pancras. I want to take the tube (it looks more direct and cheaper than the Heathrow express). Anyone have an idea how it will long it will take to get through customs, find the underground and get to St.Pancras. Which would be the fastest tube line from the airport since it will be busy with people heading to work at this time?

I read once that you can count on about 3 mins between each tube station so you can count the stops between Heathrow and St Pancras/Kings Cross. Of course there is always the chance of delays if you are unlucky. The Picadilly line is the only tube line that goes to Heathrow and it stops at StP/KC and you are right, it will be crowded with workers but you won't be the first person to be squashed on with a big suitcase and get dirty looks - been there, done that.

The airport is the first stop so you shouldn't have any problems getting on but it will get increasingly crowded then will empty out a lot by the time you reach StP/KC.

I think the Heathrow Express only goes to Paddington so that won't help you much.

Getting through customs doesn't seem to take that long but immigration varies a lot, sometimes it's very quick other times you can queue for an hour or more.
Kay

Only carry on, or do you have to wait for checked in luggage? I would believe that the 8:05 is a tight squeeze. There is only one tube line from Heathrow - the piccadilly line. If I remember correctly, it goes directly to St Pancras/King cross. The tube is cheaper - and easy to find at Heathrow. Just follow the signs once you've passed customs. The ride will take some time - 45-50 minutes.

Heathrow Express Light aka Heathrow Connect is much cheaper than Heathrow Express and nearly as fast and then a short tube ride to St Pancras.

Confirming what Vimsen says, the Piccadilly line goes directly from Heathrow to King's Cross/St. Pancras. The time given on the Journey Planner on the Transport for London site, www.tfl.gov.uk, is 53 minutes.

It can go a little faster, but you should allow up to an hour to go through immigration/passport control and claim your luggage. With the 30 minute check-in deadline for the Eurostar, the first possible train you could take is the 10:00 one.

Is Eurostar check-in 30 mins or now 45 min as i heard?

honestly don't know but am curious if it hasn't changed?

guess it would say on the ticket.

An hour from landing to reach the Tube station, an hour to get to KX/SP from Heathrow, 30 minute checkin time - 10:00am is the earliest train you can get

Thanks everyone!!! Needed to get realistic opinions. Neither Eurostar or travelcard website/email contacts were helpful. Great help making my decision with all the comments.