Just take the OS 25K map and follow your nose – most of the bridleways shown are good quality – many of them singletrack.
I would work the combes at the Holford/West Quantoxhead end. The easiest climbs (aside from the road up to Dead Women's Ditch) are The Great Road, Lady's Edge and Holford Combe. Finding the start of some of the combe descents to Holford can be tricky due to vegetation and you tend to zoom past them. The obvious ones are Lady's Edge, Weacombe and Smith's Combe – all of which are excellent.
The really cool stuff is harder to find – which is why it's so cool! For that you will need a guide.