Having a quick look, it seems that splitting at Didcot would be the best option (as advised by Dr FARTS
).
Worle - Tonbridge Super Off-Peak Return: ^64.00
Worle - Didcot Parkway Off-Peak Day Return: ^24.90
Didcot Parkway - Tonbridge Off-Peak Day Return: ^31.60
Total ^56.50
A further split could also be added at Reading, if your service calls there
Worle - Didcot Parkway Off-Peak Day Return: ^24.90
Didcot Parkway - Reading Off-Peak Day Return: ^5.30
Reading - Tonbridge Off-Peak Day Return: ^24.00
Total: ^54.20
Splitting at London (and using
Oyster▸ ) is actually more expensive than either of these options.