5 mins walk from Euston Station to Kings Cross station
Piccadilly line westbound to Heathrow no changes required
check train displays some trains go to Heathrow terminals 2 and 3
some trains go to Heathrow terminal 5
Route 1 (Terminal 4) - Incoming trains from London will branch off after Hatton Cross station to Terminal 4. After Terminal 4 the train loops around to Central (Terminals 2,3) before going back to London. These trains do not go to Terminal 5 and do not travel the other way from Central to Heathrow Terminal 4.
Route 2 (Terminal 5) - Incoming trains from London will go straight to Central (Terminals 2,3) before going onto Terminal 5 where the train terminates. After a short while the train will return to London the same way. These trains do not visit Terminal 4.
https://www.londontoolkit.com/travel...nderground.htm
other alternative is Euston to Paddington by tube circle line westbound and then Paddington to Terminal 2 and 3 heathrow express
https://www.rome2rio.com/s/London-Eu...athrow-England
Depends what time you are arriving at Euston as during rush hour the tube is a nightmare to travel on