So the biggest rollout of a vaccine ever, and you're saying we should do an antibody test on everybody before they get vaccinated?
And seeing as anyone can potentially pass on the virus, there isn't anyone who absolutely doesn't need it.
Also, if the antibodies fade after say a year, similar to flu for example, then you'd still need to start getting the vaccine next year anyway, and you may have no idea when you initially got infected so how long your antibodies will last, getting vaccinated gives you that information so you know when you need your next vaccination.
In the very short term trying to save doses for those who need it more makes sense, which is why they start with frontline nhs and older people, but it looks like most wealthy countries have ordered multiple times their entire population of doses (canada five times I think I read!) so longer term it makes more sense just to make sure everyone possible gets it to give the greatest chance of reducing the infection rate as much as possible. This also makes financial sense as the Oxford vaccine, once approved hopefully this year, costs £3 and the antibody test from one non profit organisation I checked costs £39!