Tube from Paddington to Kings Cross/St. Pancras stations

Circle or Hammersmith & City line. King's Cross & St Pancras have one shared tube station.

To add some additional description, the best underground station as given above is situated at the far end of the mainline platforms, at the opposite end from the main concourse. There are direction strips on the floor to follow.

How are you getting to Paddington?

From Heathrow on the Express then on to Kings Cross from Paddington. Might even take a taxi from Paddington
Thanks all for the info.

You can just get the Piccadilly line tube straight from Heathrow to King's Cross in about an hour, probably about as quick as the HEX. The tube line is a bit more cramped but the line starts at Heathrow so you will have seats and room for your bags and no need to change. The fare is £5.80pp, so a fraction of your planned route cost even if you are buying discounted HEX tickets.

I'll be staying near Paddington for a few days and then to France via Eurostar from St Pancras. I just wasn't sure what best direct tube was. Thanks. I've got it from here.