Is this a sign/symptom of depression or just an unrelated mental quirk?
It’s a sign of living in the UK.
Well, it’s not unique to depression, and it can just be a personality quirk, but yes, longlasting hopelessness/pessimism is among the symptoms of depression. Other symptoms include weight loss/gain, sleeplessness/excessive sleep, inability to concentrate, inability to feel much pleasure in things you used to enjoy, feelings of self-hatred, and/or the desire to hurt yourself.
Note: I’m not a doctor/therapist, just a person suffering from depression. If you want more info, here’s a definition with some of the symptoms, and here’s a depression screening test from NYU Medical School that might be useful.
Hope this helps!