The only time I flew to New York via Heathrow it took me 3 days to get there. Although I don't think Heathrow's air traffic control system crashing is that likely to happen again. Was flying to Washington last October with Aer Lingus dub - new york - washington. The flight to NY was cancelled so we were given dinner and a hotel for the night and were routed through Heathrow next morning and flown from there to Washington with BA. Have to say a much more comfortable flight with BA, free drinks, own tv screen with choice of channels. Much better than Aer Lingus. The morning flight from Dub was delayed so we only had about an hour in Heathrow but it was fine, plenty of time. Grabbed a quick coffee and straight through, no delays. And because the original flight had a 5 hour stopover in NY we got to our destination only 3 hours late, and with a €200 compensation (shopping) cheque each. the flights had only cost €350. So even if there's a screw up with connections or delays, etc.. it won't necessarily be a nightmare.