...hydraulically driven AC generators?
Rather than a straightforward engine driven accessory gearbox, doubtless using the ubiquitous variable angle swash plate so beloved of UK designers for constant speed devices, a simple hydraulic motor turned the AC generator shaft. Much like the hydraulic motors which drive high output fuel delivery pumps in some tanker aircraft.
Whereas an engine driven accessory drive (before the days of cross-drives) is clearly dependent on a serviceable engine, an hydraulic motor needs hydraulic pressure from the main hydraulic system, no matter how many pumps are actually working at the time. Probably easier to maintain constant motor speed as well.
But did the 'Vixen
really use such a system?