I would be inclined not to just go through to Cornwall but add other places as well. So, for example, you could head to a 1st base in south Devon - maybe near Totnes/Kingsbridge (but avoid Torquay area = overcrowded & traffic congestion). From this base, you could go to Dartmouth & Salcombe - both on lovely estuaries. You could also take a look at Dartmoor.http://www.southhams.com
From this area, I would then head into Cornwall to Base 2 - making diversions to visit Looe & Polperro en-route. Now, the trouble with making Penzance a base is that it is not very central. Somewhere like Truro is central for then making day trips in Cornwall. Day 1 trip could be to Fowey then Eden >http://www.edenproject.com with Charlestown thrown in if time. Day 2 trip could be to the far west = Marazion (St.Michael's Mount) , Penzance, Porthcurno (fab beach) & Minac Theatre (open air - cut into cliff), Land's End & finally St.Ives. Day 3 could be Padstow & the north coast (avoid Newquay). Day 4 could be Mevagissey & http://www.heligan.com plus the Roseland peninsula if time = St.Mawes & St.Just in Roseland. Day 5 could be Trebah Gardens plus Glendurgan Gardens near Falmouth.http://www.trebahgarden.co.uk
On leaving Cornwall to head back east, you might wish to go via the longer (& winding/hilly) A39 via Barnstaple & then into the Exmoor National Park to Lynmouth. 1 or 2 night stay in the Exmoor area would be advised as it is too much to take this route & then drive all the way to London. If using this route, be sure to visit Selworthy. (Use Google Earth to find the places). http://www.everythingexmoor.org.uk
Looks like I have used up 1 week! Try & research the places mentioned - even if you just Google "Images of ---" to see what to expect & hopefully figure out if it is right for you.