Very few, vanishingly few insurers will cover long covid as it's so new the ramifications and consequences haven't been worked out yet so it's almost impossible to forcast and underwrite appropriately.
Even fewer insurers will cover stress, especially work related stress, which initially it appears that you had (long covid diagnoses notwithstanding).
Ultimately, and you won't listen but still, neither the insurer or your employer has done nothing wrong. The employer provided the insurance as part of your employment benefits and provided the forms with which you could make your claim and the insurers declined it.
Your route was to complain directly to the insurers as a beneficiary under the policy about the claim declinature. You didn't do this. You could still do this now, as you wouldn't be time barred.
After making a complaint to the insurer and receiving a response (or after 8 weeks) you can raise a complaint to the Financial Ombudsman Service. You do not need to be insurance savvy to do so, just complaint that you disagree with the decision and they will investigate.
You have no recourse against your employer, they have done nothing wrong in relation to your insurance claim (can't talk for the rest of it as I'm not informed on employment law).
I think you will not be successful in your complaint to either the insurer or the ombudsman and you should look forward to what the next steps of your employment or benefits claim should be.