Travel times from LHR to Kings Cross station

Plan two hours using the tube. A little quicker if you use the Heathrow Express and then the tube.

If you take the Tube (Underground), it takes about an hour. Apparently slightly less if you take TfL Rail to Paddington and then the Tube from Paddington to Kings Cross, but you’ll have a bit of a walk to change from TfL Rail to the Tube. Easier and cheaper to just take the Tube in the first place.

A very useful website for London is https://tfl.gov.uk/plan-a-journey which is the website of Transport for London ( ie TfL) where you can see maps and plan any journey in London. Also, if you look at a Tube map, counting 2mins per stop from your starting station to your destination will give you a rough estimate of the journey time.

Yeah, sorry, the timing I suggested was from the Heathrow station, not including time to get off the plane and through immigration

I would allow 4 hours if you want to commit to an Advance ticket.

What I would also suggest is to book an Advance ticket to York and another Advance ticket from York to Scarborough rather than one overall ticket from London to Scarborough.

If you play around with the LNER website the savings can be significant. For example on

7 May

London > Scarborough £81 Advance; £137 Super Off Peak (walk-up fare)

London > York £22 Advance; £123 Super Off Peak

York > Scarborough £9 Advance; £20.70 Off Peak Day Single

https://www.lner.co.uk/

I think the times you have so far are good, but realize you are dealing with what we in the manufacturing world called tolerance stackup, or with timing schedule flex. Arrival is approximate, plus or minus (though I usually arrive earlier than plan on a transatlantic flight); then the variance with immigration, minutes to hours; variance in walking time to the station, if the first time, will you take a bit longer/get lost; wait time for the next train, a few minutes to 30 minutes; variance in the journey, any changes, require a walk, plus waiting again for the next train; then getting there, any time to get a ticket or find your platform. Basically the more pieces to the journey, the greater chance to time estimate has a wide variance.

Since Kings Cross is on the Piccadilly Line, which serves Heathrow as well, taking the tube would be the easiest, no changes, and probably faster than the express to Paddington, then changing to the tube to get to Kings Cross.

Overall, the tube will take an hour or so, getting out of Heathrow, with a bit of an early arrival will be 30 minutes after scheduled arrival at best, so from scheduled arrival, you are talking 1.5 hours best case (10:30 AM) to 3 hours if things get messy.

It looks like you could take TFL Rail or Heathrow Express to Paddington and change to the Tube to King's Cross.
Either is going to be a more pleasant journey than the Tube from Heathrow. TFL Rail is half the price of the Express and not much slower. I think it took 25 minutes LHR to Paddington. I used it to/from Heathrow last week and it was totally fine. It leaves from the same track at Heathrow as the Express does.
You can buy an Oyster Card and put enough funds on it for both TFL Rail and the Tube, and not have to buy separate tickets.

As stated, just the tube portion will take 1-1.25 hours. It’s an easy ride on the Piccadilly Line.

As stated, the journey, especially on a Saturday morning, shouldn't be bad. Allow between 1:15 and 1:30 hours.

That being said.....TFL loves to do work on the tube lines on weekends. So, you can find out on Thursday of the week you leave whether or not the Piccadilly line, the Heathrow Express, the tube line between Paddington and Kings Cross, will be running.

While the tube might take a bit longer, once you are on, you should get a seat on a Saturday morning, and you wont' have to walk a bit to change trains.

you can click on an upcoming date at https://tfl.gov.uk/tube-dlr-overground/status/ considerably before your travel date and get a good idea; check again just before to see if anything has been added (sometimes) or deleted (less likely).

Thanks so much everyone. I’ve got a plan now!