>>1982049I'd take the Piccadilly line from the airport: it'll take you to King's Cross St Pancras station in about an hour, and it's by far the cheapest option at just under £6; find a seat and zone out for a while.
You could also take either the Heathrow Express non-stop train to Paddington - or the slightly slower Elizabeth line train to Paddington - and then take the Circle line from Paddington to King's Cross. It'll be more expensive (roughly £25, sheesh), but it'll be a little faster and a little nicer than sitting on the Tube for an hour.
Whatever you do, make sure to grab an Oyster card at the airport: put a little cash on the card, and whatever you don't use you can get back if you return the card later. Or just keep it as a souvenir lol.
