A hypothetical scenario here, but how would a Highbury & Islington - Heathrow Rail work with Oyster or contactless?
There are various routes that could be used such as:
Victoria Line to Oxford Circus, Central Line to Ealing Broadway, then London Crossrail
Overground to Stratford, then either Crossrail direct or Central Line to Ealing Broadway, then Crossrail
Overground to Willesden Junction, then Bakerloo Line to Paddington, then Crossrail or the Express
Overground to West Brompton or Kensington Olympia, District Line to Earl's Court, District to Ealing Broadway, then Crossrail
Overground to West Brompton or Kensington Olympia, District Line to Earl's Court, Piccadilly Line to Heathrow
Northern City Line to Moorgate, then Crossrail direct
There may be other variations for this journey.