this click here shows the great circle route (shortest distance between two points on the earth, not the same as a straight line on a flat map). After "two hours over the atlantic", Heathrow would almost certainly be one of the closest suitable airfields.
into American airspace, and you can't just dump a 747 into any airport you choose, unless there's a real emergency. All passengers inbound to American airports from international destinations are checked by US Homeland Security before the aircraft leaves its departure airport. If the US entry authority spots someone who is a problem it will contact the inbound airline and ask for the passenger to be denied a boarding pass. You can argue until you're blue in the face, but you won't fly if they don't like the look of you. On this occasion the US authorities obviously passed this passenger to fly, but spotted something after the flight was well into the cruise.
I can imagine the reaction if an aircraft contacted Portuguese air traffic control and said 'we've been refused entry to American air space because we have a suspect passenger onboard - is it OK to lumber one of your smaller airports with the problem?'
If I look out my windows (skywards) I can see about half a dozen jets passing above.When the Lockerbie disaster occurred,I formed the opinion I was lucky the bomb hadn't exploded a few minutes earlier as the wreckage could have landed on my home town.I looked up the flight-paths to the USA and put simply,the aircraft take-off in the UK and head north until almost the scottish border where they turn left.