If they are fortunate enough to get that OA approved, they would configure any new order appropriately. It would be crazy to specify units which will be fully utilised on their existing routes suboptimally for those routes purely on the off chance that they get the chance to run a service down to the south coast.
To some extent, if the trains have modular power packs, the different units can be designed to hang on the same brackets and be swappable.
They have a 5car train with 4 units: transformer; diesel; battery pack; and second diesel or battery. So does that leave space under 5th car. There are already inverters to convert dc from the battery for traction.
Not very difficult to add some third rail pick up shoes, and use the 750v dc supply in South of England. A bit of power electronics, through the inverters and got dual voltage. Even easier if the batteries run at approx 750v as don't need to convert the voltage. Admittedly might need higher amp rated inverters, but that is small marginal cost.
The solution exists, not just GC, for instance GWR run services to Gatwick, Weymouth, Portsmouth (all third rail electrified) so are unlikely to want BEMUs that cannot use third rail. If you have the dc batteries and inverters already fitted, then allowing third rail add on is relatively cheap.