Big non-event. RAF E-3 AWACS had a hydraulic issue and returning to base.
The plane has multiple hydraulic systems & manual back-ups etc.
The 7700 squawk is to grab the controller's attention it does not mean everyone is doomed to die.
https://www.express.co.uk/news/uk/1370412/raf-jet-emergency-Sentry-ZH103-AWACS-aircraft-RAF-Waddington