It's not about 'threats', it's about politics.
The Russians are demonstrating a capability. Our response is for all the reasons Leon gave plus it is also a political statement. 'We know you are there and we will watch'. All the talk of subs and fighter escorts are best left to a Tom Clancy novel. There would be a whole lot of political sniping/posturing before we got to that.
Ukraine May well continue to escalate, we shall see. As to intercepting Russian Bears off our own Islands, we will keep doing it as part of our NATO commitment.
I would not mind betting that Joe Public would rather see the RAF doing this sort of work than dropping bombs in far off deserts, no matter how good the argument. And, let us not forget, the Military is actually run by politicians, NOT Admirals, Generals and Air Marshals.
Regarding intercepting Civil Aircraft, that is part of the UK Anti Terrorism infrastructure and so the two should not be confused.
A final thought: abolish QRA and there would be two main consequences. First you could kiss goodbye to the best part of two Typhoon Sqns, plus the ground based AD C2 systems, as QRA is a core tasking and, second, the political fallout would be considerable. How could Dave and his chums bang the gavel of righteous indignation around the world if he headed a Government that was not even willing, or capable of policing it's own skies and the approaches to it?
Finally, and most importantly, I have not played 'Ukkers' since leaving the RAF.