I got to Cheddar on day 1 of my LEJOG without anything particularly hilly and without using the A30. So unless you're trying to break some record there's no need to use that road. Not using the A30 didn't add many miles to my route anyway. And FWIW I did use some A-roads (a lot of A6 and A9 in particular), but an hour's research on the internet made it pretty clear the A30 would be unpleasant and that there were really pleasant alternatives. I'd not cycle on the M1 (even if it was legal) so I wouldn't cycle on the A30(M).