If you can do early morning, and i mean early morning, in the car by 5am Fri at the latest, you'll be in Dorchester for a slightly late breakfast, Fridays up to about midday can be surprisingly quiet, it's midday when the lemmings flood out and drive headlong to oblivion.

If you can't do early then i'd leave around 6pm Friday evening or mid afternoon Saturday.

Probably not what you wanted to hear.

If you leave the Midlands after about 3pm on a Saturday in summer it's a surprisingly fast run down the M5. Most people going on holiday in Devon and Cornwall have to arrive before 6pm so can't leave that late.

If you can't leave at that time, as you're going to Dorchester rather than Devon or Cornwall, you don't need to go on the M5. The 'Fosse Way' route (A429) is surprisingly free of traffic, and after Cirencester you can carry on down past Chippenham to Dorchester on the A350. The A350 is full of lorries during the week, but not too bad at weekends. Don't expect to average much more than about 50 over the route as it's mostly single carriageway, but you won't on the M5 on a Saturday at that time of year anyway, and the Fosse Way route is about 30 miles shorter.

be up before the sparrows and stop for breakfast in the middle of the morning rush hour ...