This is what we have found works well for us for similar journeys down to France. If you can be organised enough to leave earlier then brill but this was on the basis that we couldn't get into the cottage until 12noon on Saturday but didn't want to spend all day on Saturday travelling.
Early and large lunch at 12pm, then straight in the car for long 2 hr nap.
If you are lucky you'll get to 3pm before stopping. Quick pit stop at services for nappy and snack. Back in car within 20 mins
Afternoon of serious ipad time, stories and toys in back of car. Break at 5.30, 6 at latest for food and exercise. Allow 1.5hrs min.
7.30 - milk in car seat, story and sleep. Drive until 10pm, then break in B&B overnight.
Gentle start with nice breakfast and a walk around in the morning. No more than 2 hrs to destination. Get there, have lunch and move in.
Coming home we just drove straight through and arrived home around 2-3am and dealt with the pain on Sunday morning. Its easier coming home to your own house.
Download a serious amount of stuff onto an ipad. DD was 15 months when we first did this and DH emptied most of the contents of the kitchen drawer into a bag before we left. Handed her a spatula, milk whisk, wooden spoon, broken computer mouse, silicone pastry brush, old remote control etc etc every 10 mins to be waved about and explored. The mouse on a cord was a particular favourite for some reason.
It's worth lining the car seat with a disposable changing mat in case of poo explosions but also bringing an entire change of clothes and wipes in case of car sickness. Some kids don't do well with ipads - causes car sickness to watch stuff so worth trialling before you go anywhere. THe smell of sick for 8hrs won't make anyone feel good.
I can't speak for the West Country but we've stayed in some lovely B&Bs on the road to France. It's really nice to get up at a relaxed time on Saturday morning and stretch the kids legs in their gardens knowing you are 5 hrs ahead of the rest of the world setting off. If you leave enough space in the car, you can also do a supermarket shop en route just dumping it all in the car for the last mile or two and not have to leave again once you get there.