As people have asked, some points on the ethics question.
First off, pragmatically speaking, we docs will pretty much always sign off anyone who asks for it. The alternative has too much potential to cause stress and argument for us, and even in better times its not a good use of our time to fight some fights.
Re the diagnostic criteria, they're entirely based on subjective report of symptoms. If someone describes persistent and severe mood disturbance they meet the criteria.
Re accepting treatment, I think that it's reasonable to decline medication if a life change achieves mood improvement. Depression =/= needs meds. Re counselling, that's a limited resource, but basically you get a phone number, a 3 month waiting list, then some group zoom calls. Unless you're under 18 then you get a list of charities who might help. Actual psychiatrists are for patients with psychotic illnesses or self harm. Depression isn't something the NHS properly provisions for.
My own view is that people should be allowed to take time off purely on their own declaration, but that sick pay durations should be limited by law, after which employer obligations should fall away. I don't think doctors should be involved until there's need for dispute resolution.
For the specific issue of covid risk causing anxiety, my opinion is skewed by the fact that my own workplace carries more risk than theirs, so part of me thinks "if we can do it, so can you, and if you can't, get out of the job and let that wage go for someone who can."