We have been told that

The X1 Bristol to Weston-super-Mare, X39 Bristol to Bath (First Avon & Somerset) and X38 Exeter to Plymouth (Stagecoach Devon) accept passes.

We have also been told that

all express bus / coach services operating in the Westcountry (Gloucester/Wiltshire southwards) which have an "X" number, allow free travel to bus pass holders. I think you will find that this applies pretty much to the rest of the country too.

The only express services Westcountry Exptress services that don't accept bus passes are...

- National Express services
- Megabus services
- Berrys Superfast services to London