Why South Georgia was not retaken by a token force

Was South Georgia retaken only by the Royal Marines available on the flagship?

No. A substantial commando force was available, while landing-craft capacity and helicopter losses limited how quickly it could be used.

The force assigned to retake South Georgia included a company from 42 Commando, equipped for the operation. The limiting factor was how many troops could be moved ashore at once. Helicopter losses tightened that transport bottleneck further. The distinction matters because the small initial landing did not represent all the troops available for Operation Paraquet.