Yes, they can.
Even tesco can bar whomever they want without having to give any reason.
Of course if they didn't serve someone and said they don't serve ethnic minorities, that would be illegal.
But atm, covid would appear a good reason to refuse entry.
What I meant was; if shopkeeper refuses someone because that someone is suspected of having covid, then IF that someone has any protected characteristics that someone would claim it was discrimination because of those characteristics.
They can yes but they may get in the doodoo with management.
The same managers that allowed the bulk buying to continue so long only think of ££ and never the poor shop worker.