I recall Worldways used to operate EDI-PIK-YYZ on the Tristar and the DC8 when I was a kid but since the end of that policy under Cecil Parkinson there has been nothing to stop them if the market's there,
Am not sure what part Cecil Parkinson played in the matter but it was Air 2000 c1990 which put an end to the ludicrous idea that transatlantic flights could operate only from Prestwick. (Although the silly 7-8 minute lob across Ayrshire was quite fun for the pilots!)