Plymouth does have a regional airport with links to London, Bristol, Manchester, Leeds and Bradford. Routes are operated by Air Southwest. Additionally Devon has a second airport in Exeter (approximately 40 minutes drive from Plymouth). Exter airport has links to several countries and more details can he found here.
Plymouth can be reached from by rail from all major cities in the UK. Tickets can be purchased in advance at thetrainline.com
You can also get to Plymouth using the roads. If you don't fancy driving yourself there are bus services available from most major cities in the UK. One of the most frequent is National Express
The final option for those who live on the continent is by Sea. Brittany Ferries run up to 2 sailings per day from Roscoff (France) and 2 sailings per week from Santander (Spain)