I've moved bees several hundred miles, including 2-3 hour ferry crossings, all with no problems or worries in the back of a standard saloon. Most of the points have been covered but I'd add :
Travel screens if it's warm (though it sounds as though you'll be doing this very early in the season)
Stand the hives on wooden battens to give airflow underneath
Strap the hives to lash-down points in the boot (most cars have these now)
As Heather say, frames in the direction of travel
Bee suit on the passenger seat, unless you're feeling very nervous in which case wear it
Make sure there are no stowaways under the OMF
I've always pinned mesh over the entrances and never had problems, but I know others who say bees panic if they can 'see' out. However, if you use foam wedges I'd put mesh over the top
I've seen a contributor on here using mesh curtains to wrap everything in, which seemed very sensible
Classical music or radio 4 only ... they get very agitated by popular music
Finally, I've not moved them in cold weather when they're clustered. Perhaps someone else can comment if this is sensible or not. I suspect the vibration might disturb them and the car is bound to be quite a bit warmer ...
Enjoy Somerset!