They should still detect it. Particularly if you take multiple antigens. They only give advice on not using them if you have symptoms so that you go through the official testing channels.
My Mrs did an antigen on Tuesday night which told her negative, but as she has symptoms, she had to go for the PCR test. Couldnāt get one til Thursday. Result was yesterday and negative, but as she couldnāt go into work Wednesday - Friday, she now has to go in today and tomorrow.
My wife for past two days was dying. She was serious āman fluā sick. Loads of lemsips etc. Negative antigen test this lunchtime. So after we went to the local where she drank 4 pints of porter. Sheās now sleeping like a baby and the snotting has stopped.
Guinness is good for you.