For example: London-Exeter route Honiton is £71.60, and via Taunton
£103.50

If I had a via Honiton ticket and wanted to return via Taunton could I
excess it for half the difference? Or would they want the full £31.90

Similarly, in reverse, can I excess a "via Taunton" ticket to be "via
Honiton" for a negative amount [in practice I wouldn't expect a cash
refund, just the ability to travel for no extra charge].
--
Roland Perry