Head for the South West on the M5 from Bristol to Exeter, then the A38 Devon Expressway to Plymouth. Drive time from London is around three and a half hours.
Plymouth has three park and ride schemes on the A38 and A386 main access routes to the city : http://www.plymouth.gov.uk/homepage/transportandstreets/publictransport/parkandride.htm

National Express coaches from London Victoria take four and a half hours :
www.nationalexpress.com
Megabus coaches from London take about five and a hald hours:
www.megabus.com
Stagecoach Bus e.g. from Exeter to Plymouth
www.stagecoachbus.com