Cornwall to Edinburgh

I would first outline what I want to do in each location and decide how many days in each.
I prefer to travel by train but we did have a car for the week we were in Cornwall. ( younger than and more comfortable driving) . Are there places you want to go between London and Edinburgh or Cornwall and Edinburgh.
Don't know the flights but I would try to fly home from Edinburgh and then into London.
If I was doing this trip.....Fly into London. Spend x days. Train to Cornwall. Spend x days and then train to Edinburgh .

It would be helpful to provide more detail about your plans , i.e. How long is this trip in total ? Once you arrive in Cornwall , are you planning to hire a car ? Seeing sites in Cornwall really does call for a set of wheels . ( I've made several trips there , and always had a car . What are your plans between Cornwall and Edinburgh ? Driving that route is brutally time consuming , rail would be a better choice , are you stopping elsewhere along the way . ? From which airport will you depart for your return flight ?

That is plan A, looking for alternative ideas as this forum has never let me down yet! I am wanting to have car least amount of time and only during Cornwall part. I’ve been to London but might spend one day there. I’m happy to train between cities.

There are seasonal flights from Glasgow or Edinburgh to Newquay so you could fly. But most folks wouldn't bother as it would be too time-consuming unless you're in the UK for a longish period.

To clarify, flying into London and out of Edinburgh. Unless better options. Need help on logistics of travel itinerary as I want to get airfare first.

Im open to flying to Edinburgh only if faster than train. Only want car for days in Cornwall. Train from London to? To pick up car?

Potentially: fly to London. Stay at least a day. When you’re ready to go to Cornwall, take the train from London Paddington to Penzance (if you want to explore the far west - alternatively somewhere more central like Truro.) Rent a car. Enjoy Cornwall. Either drop the car back to where you rented it and take public transport to Newquay Airport, or potentially drop the car back at the airport. Then fly to Edinburgh from Newquay. Enjoy Edinburgh. Fly home.

The train from Penzance to Edinburgh would take about 11 hours, which is why I suggest flying.

Have a look at train times and prices here. The tickets called Advance are much cheaper. https://www.nationalrail.co.uk/

There are two direct trains a day from Penzance to Edinburgh BUT no-one would ever do that journey through as there are no advance fares so it costs a whopping £264. Even splitting the fare into multiple parts you can't get that down to a remotely sensible figure.
You can get it down to about £120 (maybe £100) by splitting the fare at Bristol, and changing to Avanti West Coast at Birmingham.

The way I do it, several times a year (from Carlisle) is by National Express. I leave at 1320 and arrive Penzance next day at 0700. It costs me £15. It would be the same price from Edinburgh. But no-one on this forum will approve that.

By train the sensible way is train to London Paddington, then Kings Cross to Edinburgh. The even better variant on that is the Riviera Sleeper Cornwall to London. Use the lounge in the Royal Waiting Room at Paddington to freshen up, then early day train KC to Edinburgh.

I will not fly domestically, period, for environmental reasons. But the seasonal flight from Newquay is the sensible way to go. Check in time at the small Newquay airport is not long, and the fare including checked luggage is usually £50 or less.

There are also year round flights on Loganair from Exeter Airport, in Devon- usually around £100.

Truro and Penzance railway stations have car hire depots at the station.

Both Ryanair and Loganair have seasonal (summer) nonstop flight from Cornwall Airport (Newquay) to Edinburgh. The flight is around 90 minutes.

The Ryanair flights can be as low of 25 GBP before the extras.

Really great advice people, keep it coming! Many thanks!

Are you intending to spend any time in London? If not, you can fly into Newquay relatively easily from Atlanta via Dublin. then fly NQY-EDI and a direct flight home from EDI.

I once drove in a rented (hired) car from Cornwall to Edinburgh. From there, we took a train back to London for my return flight. I don't remember that drive as particularly memorable one way or the other, but we did stop at Warwick Castle, a picnic lunch on Hadrian's Wall, and stayed at a cute B&B (real B&B, not an "air") in Northern England.