London to North Cornwall, which way would you go? and where to stop off with 3 year old

[domeafavour]

I've done this journey a couple of times and always gone m3, then a303. never had any problems.
But DH wants to go m4 and m5.
Anyone do this journey often? Any pitfalls?
And any recommendations for a stop off on the way, either 1/2 day or even somewhere for the night?

thanks