>>> The case against Abdelbaset al-Megrahi, the only man so far convicted, looked dodgy
It looked very, very dodgy indeed. There's plenty of evidence to suggest that the bombing had absolutely nothing to do with Libya at all.
To answer DTC's question though ("But why not in Glasgow or Edinburgh as the terrorist incident occurred in/over Scotland.....?"), the US government claims jurisdiction over what goes on onboard any aircraft operated by a US airline, or which is flying to the USA, other than when it's on the ground in another country with its doors open.
So, as far is the US government is concerned, if you board a British-registered aircraft, operated by a British airline, which is due to fly from (say) Heathrow to New York and you then commit a crime while that aircraft is simply pushing back from its stand at the airport, you can be prosecuted under US law. (That doesn't, however, mean that you can't be prosecuted under English law, as both countries can claim jurisdiction under such circumstances).