No Bi-Di north of Yate, the northbound line is signalled for wrong-direction moves to allow freight from Tytherington to depart straight out and avoid extra shunting. As the crossovers are there for access to the Westerleigh Oil terminal, the route is also provided for a southbound train from Gloucester to use the bi-di facility. However I'm not aware of any regular through trains being planned to use the line wrong-direction. I've seen plenty of trains use it though on the live data maps, when late running stopping trains are held at the junction to allow (usually) a XC to overtake.
Westerleigh Jn to Bristol Parkway (in common with much of the GWML between Didcot Parkway and Bristol Temple Meads) is Bi-Di but reduced capacity in the wrong direction.
Not quite.
This main line has patches of bi-directional (reversible) signalling, but otherwise is only unidirectional.
Between Pilning and Patchway Junction the up line is reversible. The down line is unidirectional. However, there is a reversible signal on the down line just east of Patchway Station platform.
Between Patchway Junction and Stoke Gifford West Junction, the up line is unidirectional, the down line is bi-directional.
All lines through Bristol Parkway are bi-directional.
Between Stoke Gifford East (Bristol Parkway) and Chipping Sodbury, both lines are only unidirectional.
Between Chipping Sodbury West and Chipping Sodbury East, the Up line and the goods loop are bi-directional, the down lines is unidirectional.
Between Chipping Sodbury East and Hullavington, both lines are reversible.