I was speaking to someone yesterday who's mates had all bought the antibody test thinking they'd basically be able to go back to normal (i.e. meet mates again) if they tested positive.
AFAIK there is no clear evidence you build lasting immunity. Seems very reckless that they're being sold on a few websites without any real warning on this.
The problem is, what do you do with the result? You'll (and your employer) still have to comply with all the government guidelines, as if you're at risk. I'd rather wait until prices come down or it's available on the NHS.
I can understand if you're for example looking after extremely vulnerable people and are worried you'll bring it back home, so more for your own piece of mind. But then you still have to be careful because of all the things said above about immunity etc. (I do believe you get some sort of immunity, but it is unclear how complete that is and hiw long it will last (it will be afew years at least I think).