I'm trying to upgrade a S class (PE) ticket using miles, to G class (UC) on the LHR - SFO route in July outbound and the LAX - LHR route in August inbound. Currently VS don't have any showing as available. Is there somewhere I can get this information without chasing them daily? Or leaving it until the last minute on the day of depature.
No.
You need to keep checking then nab them when you see them.
But you don't have to chase/phone VS though you can check yourself by doing a dummy UC spend miles booking and when/if they do appear that's when you need to phone.