I would say if its taxable for the employee then it would be taxable for the family as well.

Yes, I believe antibody tests would be a taxable benefit for both. Antigen/PCR tests are not.

I am not sure I see the benefit of offering antibody tests though as they are pretty inaccurate and do not provide any reassurance that someone who has been infected is immune, but I guess some people may be curious and find some comfort in knowing either way.

Would it not be treated as a “trivial expense” and therefore not part of a BIK package? My work offer reimbursement of the flu jab and HMRC state to treat is as an expense, not a benefit. If the employee is paying for their family to have the test then that is not included in the employees P11D if you are required to declare it. If it costs less than £50 a test, it’s called as “trivial benefits” so not BIK