Doesn't the Picadilly line go direct from LHR to St Pancras?
When we left London last month, I just shoved the kids on the tube at Green Park and off they went to LHR. It was the same train we took from the Eurostar terminus (at St. Pancras) to Green Park.
Is it faster/cheaper to take the HEX to Paddington and then a taxi to St. Pancras? Or am I missing something?