Cheddar Gorge, not to far out of your way, then carry on down the A roads through Wells and turn left at Shepton Mallet on the A37 for Ilchester, I would give Bristol a wide berth due to the traffic but if you must come off the M5 at Avonmouth, head along the Portway under the Avon Gorge Bridge, Brunells, and follow the signs for the SS Great Britain, from there back towards the Cumberlain Basin and turn left for Cheddar.