i wouldn't recommend a flight to Manchester, which lies in the North West of England and would be a very long drive to Cornwall, or a long train trip, probably requiring a change of train. The best international gateway would be one of the London airports, or Amsterdam. It's quite a long drive from London (either Heathrow or Gatwick) and the journey could take up to 4 hours (230-250 miles) to get to the entry point of Cornwall. Possibly you could fly from Amsterdam to Exeter (although some flights require a change of plane at London Gatwick), or better get a flight from London to Newquay, Cornwall's main local airport.
There are some direct flights from the States to Bristol from Orlando.....don't know how easy it is to get there from where you are.
Used to be some from NY but can't find them. Otherwise it is via Amsterdam or Dublin.
Personally I would go non stop Atlanta to London Heathrow - because you will be tired I would take the train to Exeter, relax & watch the English countryside change as you travel, rest overnight, enjoy Exeter and then drive down to Cornwall.
I would be another to recommend flying direct to Heathrow, and then either coach or train to wherever you are staying in Cornwall and pick up a car there.
Brilliant and so quick! Cheers! My advise would then be: Fly to Heathrow and make your way via coach to Gatwick where you pick up the Newquay flight.
Wouldn't there be a problem with baggage then though? Most flights from the US have a much more generous baggage allowance than Fybe, something for the OP to bear in mind.