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.

Heathrow to Kings Cross via Piccadilly Tube if you haven't already purchased HEX tickets. Significantly cheaper than buying HEX tickets at LHR.

I do think HEX is more comfortable, but personally take Piccadilly tube whenever I land at LHR and want to get to Kings Cross. There will be seats from LHR. Worth the slower ride and less comfort to avoid the Paddington to Kings Cross extension for me. (I might feel differently if I traveled with large suitcases, but I always manage a carry-on on the Piccadilly line fine.)

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.

^^^ Got it! Aren't we helpful with more info than you ever needed!!

I, personally, would pick up a taxi like you are considering. Indulge a bit! No fun looking for elevators or dragging bags around tube stations (my opinion....)