Getting from southern England to Normandy

Well, there are currently no airlines operating out of Plymouth airport, so the nearest would be Exeter. There appears to be a weekly Flybe flight from there to Rennes, for around €50. That's not particularly close to Caen, so a few minutes spent with google suggests that flying in to any closer airport means going to Deauville, and the only direct flight to there that I can find seems to be from London City airport. Brittany Ferries operate the Plymouth to Roscoff route. For foot passengers the crossing is around £29 (€35 or $44), or £39, depending on the day of the week. There is an 08.00 departure from Plymouth, arriving 15.00, but if you got one of the overnight crossings and you wanted a reclining seat that will cost an extra £5 and if you want a cabin then that's another £55. Roscoff to Caen is 340 km and about a 4 hour drive. Brittany Ferries also have crossings from Portsmouth to Caen. Prices are about the same as Plymouth-Roscoff for foot passengers, so you could get the 14.45 departure, arriving 21.30 for £29. Plymouth to Portsmouth is about 5 hours and involves one change of trains. The National Rail website is showing Advance fares for June from £18 ($27), departing Plymouth at 07.48 and arriving Portsmouth at 12.54. That gives you time to get to the ferry terminal to catch the 14.45, with the total cost for Plymouth to Caen of £47 / $71. The only thing to remember about Advance train tickets is that they are only valid on the particular train you book. Miss that and you pay the full single ticket price, plus a penalty if you board a train without a valid ticket.

Or you could get the Flybe flight from Exeter to Paris CDG from £60, then it's a 2 hour €15 train ride from Paris St. Lazare to Caen. Plymouth to Exeter takes about an hour on the train, then a 5 mile taxi or bus ride to the airport.

Thanks for all the detail, Kevin!
Do you think it would make more sense (time and money) to take a train back to London, a flight to Paris, and a train to Caen? Is it the same amount of time overland as across the channel?

I think if I was going to fly at all, I'd go with Flybe from Exeter to Paris. It's an hour from Plymouth (plus taxi time from the railway station), compared with a minimum of 3 hours from Plymouth to London Paddington, then you would have to get from there to whatever London airport you were flying from. Any money you saved on the plane ticket would be lost again on the additional train fare to London.

The 11:30 Exeter Flybe flight arrives at CDG at 14:00. Allow up to one hour to get through immigration/passport control at CDG and another hour to get to Gare Saint-Lazare. Trains leave Gare Saint-Lazare at 16:45, 17:10, and 17:45. The €15.00 fare for Paris (Gare Saint-Lazare) - Caen is a Prem's fare for the cheapest advance purchase ticket. The ticket is for a specific departure date and time and is non-exchangeable and non-refundable. If your plane is late and you miss the train, you will be out the price of the ticket. The standard 2nd class fare is €34.80.