Madley to Weston Super Mare would be my suggestion.
448 Madley to Hereford (Operated by Newport bus from 30th August.)
66 Hereford to Monmouth (Newport bus)
65 or 69 Monmouth to Chepstow (Newport Bus)
T7 Chepstow to Bristol (Newport Bus)
X1 Bristol to Western Super Mare (First WofE)
Granted, the 448 only runs on Tuesdays and Thursdays and may not always be allocated an electric, and the 66 can sometimes not be electric too, but possible with sufficient luck.
I make that 84 miles without coaches involved at all. You could probably make it closer to 100 by instead taking the A1 to Bristol Airport from Bristol Bus station, changing to an A3 there for WSM and if you get lucky get the 7 on to Worle if that gets an electric.