Travelling on a Wednesday is best in the Summer if you can.
I once tried getting to a wedding in North Devon - there was a major crash on the M5 Avonmouth bridge and the road was shut for over 6 hours. Some people tried getting through Bristol City Centre and I was told it took 4 hours. We turned back and missed the wedding.
To get to the West country from the North, West Midlands, Wales you really can't avoid the M5 - hence big queues in any sort of holiday period. A lot of peole from the West Midlands in particular head that way.