An overnight stop, even on a campsite is much much better than a long drive down from North Staffs on what can be a VERY busy/congested motorway that often gets blocked due to caravabs "falling over" with the inherent dangers of fatigue and dozing off. Sadly such occurrences are not unheard of on that road, especially South of Taunton. Ask yourself why that might be.
Better to arrive late than dead on time being my motto these days.
Another alternative of course is to traveling in the early hours when traffic volumes, and the presence of trucks are much less, that way you can maintain a very good average speed over long distances. But you do need to have had a good sleep first or you run the risk of nodding off due to boredom.