What made you notice it had dropped to 120Mbps - have you been speed testing again?
If the lights aren't on 24/7 then the SRA that GFast uses should see normal speed restored when they are off.
Since you have said in the past that you are going to go back to a 40/10 FTTC service once your GFast contract expires then I'm sure you can manage with 120Mbps until the festive season is over (accepting that it is annoying when these things happen).