I think it depends who the insured party is. If you are the insured but the payout is routed via your employer I think is taxable.
If your employer is the insured and they would have still paid you the lump sum even if the insurer rejected the claim (ie they were simply insuring against them having to pay out) then I would expect it to be tax free.
A clue might be if you were taxed (eg on p11d) for LOL insurance or if you were given details of policy terms or insurance certificate. If either is true then YOU were the insured party, not your employer.