You're welcome.

If you have an Oyster card already - and it sounds like you do - then just take the Underground to King's Cross and buy the correct paper Travelcard there. An off peak single from LHR to KX costs just £2.40, there is an escalator then lift up from train platform to street level - and the ticket office is open from about 0500 - 0130 on weekdays.

You can take the Underground from KX to Baker Street for the Jubilee line to West Hampstead. If memory serves, the walk from the westbound Circle line platform at Baker Street - platform 6 - to the northbound Jubilee platform is a long'un (trudge, trudge, trudge) with the walk from the Metropolitan line platform - platform 3 I think - less of a hike.

But if you want to go to Kentish Town, you can take either the 46 or 214 from KX; this link shows the bus stops.