I've worked in Cornwall plenty of times and that dreaded A30 can be an arduous slog in the summer months, especially at bottlenecks where it becomes single carriageway. Some of the locals and tractors don't drive anywhere fast so patience is required. As others have said it's best driving through the night, but only if you're an experienced driver stopping of for a toilet and coffee break or two. Not easy driving on unlit roads at night. I'd be tempted to fly and hire a car there if I went on holiday but for work I had no choice because of all the gear in the van. In light traffic without a break Manchester to Falmouth is possible (legal speeds, light traffic) in about 5 n' half hours so I'd be looking at 6 n' hours with a couple or so breaks.
I liked Falmouth best because I did a 3 day 4G rig on the roof of Trago Mills. Getting paid good money to have Falmouth's lovely scenery around me in nice sunny weather. Bliss.
Went in Rick Stein's there, good food, small portions and expensive. If you want excellent fish and chips go to the chippy next to Trago Mills.
I've forgotten about some of the small towns and villages I've stayed at but as others have said Cornwall (and Devon) are great counties to holiday in set amongst some of the most pleasant countryside in England. But it gets very busy and expensive in the summer months, especially school holidays. Been 4 years since I last visited, must go back but next time purely as a holidaymaker.