Since we will be staying at Grosvenor House on Park Lane, which EasyJet airport would be the easiest to get to for the flight to EDI?

I look at the train price for our departure date of Sept 10 - GNER has not released any price other than something in the hundred's of pound range. The lowest standard ticket is 25BP but I dont see any in July thru August, the cheapest I saw availability during those tested period was 40BP.

EasyJet flight is from 16 to 21 (before tax and surcharge of credit payment of course it would add another 5bp)