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There is a direct underground train from Kings Cross to LHR.
I find it no trouble at all though I mostly go via AMS.
Aye ye ol' Piccadilly line 20 commuter stops to Heathrow deep tunnel tube train again nowhere to safely store luggage and £6 one way for an hour still of bumpy uncomfortable tiring travel and still the risk and adventure of standing for much of the way.
Still great.
Going via Amsterdam though understandable even if the KLM B77W is little different to the EK cattle class frames (just 3 seats difference !)