I recall Easter weekend 1974 when the West of England main line was closed (I think at Whiteball) and both London-bound and 'Cross Country' services were diverted over The Mule to Yeovil Junction and on to Castle Cary ('Cross Country' services then to Westbury and Bath Spa). I am not sure about 'Cross Country' services bound for the North East, but both
1M91 (Plymouth to Manchester Piccadilly) and
1V72 (Manchester Piccadilly to Plymouth) were also diverted via Cheltenham Racecourse and Honeybourne. It would have been a long journey for anybody travelling to Birmingham and beyond on those services. I am not sure where the driver changes would have been.
Another matter that needed taking into account following dieselisation was traction knowledge. This was not an issue in steam days because a driver was expected to drive whatever he was given. As an example in the 60s nobody west of Bristol had traction knowledge of Peaks (Class 45 and 46 post-
TOPS▸ ), so if one was rostered on a train to Plymouth it as imperative that the same driver brought it back, otherwise somebody would need to be sent on the cushions down to collect it.
In the scenario you mention the Honeybourne issue would have been covered by Bath Road and Saltley route knowledge but would need a conductor over the LSW. There may have been Laira men who had route knowledge of the LSW, but they would need a conductor from Pen Mill to at least Castle Cary