I would set off on Friday !!!
I assume you will going down the M5.
Well as you can see if you look at a map, all the traffic going to Devon and Cornwall for holiday goes down the M5.
The traffic coming from Wales, and from London on the M4, and from the North down the M5, all merges around Bristol.
On a Saturday IT IS A NIGHTMARE.
It is so bad that when I went to Devon on holiday a few weeks ago I booked a hotel in Torquey for the Friday night and travelled Friday to avoid the Saturday traffic.
But even then there were THREE crashes on the M5, two of them overturned caravans. We had to turn off the M5 at Bristol and went the rest of the way via the A38.
Friends of ours who were were staying with travelled down on the Saturday, and they said the M5 was like a car park, with MILES, and MILES, and MILES, of slow moving traffic.
Your friend whose check in time is 2pm (I guess not as you said 2AM) who is leaving at 3am, has the right idea, but he still may not miss the jams.
So to sum up, the M5 on a holiday Saturday, is a nightmare.