Is it just GWR▸ though? Network Rail control the signalling and therefore the option to run reversible on the up line. Can GWR insist on it?
Network Rail will have staff on sight who will travel up and down on the trains when rough weather is expected. They would normally be the first port of call when it comes to closing the down main.
Other than that, if Network Rail receives reports from drivers via the signallers stating waves are crashing over, debris or potential that may give them cause to close the down main.
GWR Control would be unlikely to demand that course of action.