I don't think the expectation that the aircraft would have been out over the Atlantic on a more usual track is correct, as it would still have been over Ireland on a normal day.
Like many transatlantic flyers over the years, I've noticed a wide variety of tracks. There was a time some years ago when on LHR-MIA we went over Newfoundland more often than not. More recently some more geographically direct routings seem to apply. ETOPS is nothing to do with this, and the Pan Am was a 747 anyway.
Like many here it seems, I can recall the Lockerbie day. I was passenger on a BA 747 LHR-LAX which took off some hours before, so of course being on board we knew nothing. On arrival at LAX it was something that had happened to a flight not connected to there, so not an issue. Back in the UK however all hell had broken loose with my colleagues and relations. This is where you discover all sorts of myths, like "there's only one plane a day to America", or that I "always go with Pan Am" (in fact I never ever used them).
It wasn't helped by the fact that the flight number was, IIRC, through LHR-JFK-LAX, and also I believe the flight deck were from the Pan Am LAX base, as I recall when their details appeared in the press they had Southern California addresses. This probably led to some misreporting in the UK that it was headed to Los Angeles.
It was a few days before Christmas and it had been difficult even a month beforehand to get seats. Of course my flight, as most other transatlantics that day presumably, was full. I have always wondered why the accident aircraft was only just over half-full.