There is also the X8 as a decker is too high to go under Nailsea and Backwell Station.
Banwell and Axbridge are the main reason for not operating deckers on the 126, though there are other parts of the route where it would be a bit tight. When Weston depot used to send the VR and Limp-a-long open tops to Cheddar for the George tour, they had to go to Edithmead then back down the A38 onto the A371. If a decker is needed to go from Weston to Wells now, it has to go to Bridgwater then via Glastonbury and Street.
I would presume if an ALX400 would fit, the only other deckers capable of going under Bitton Railway Bridge Are the Scanias.
Deckers do still operate under there for the Oldfield School 37A journeys operated by Bath, with the main service operated by Marlborough Street B7RLEs or Street****es.