Obviously it will improve over time
Every one of the innocent people incorrectly flagged will be arrested. Which means they will be photographed, fingerprinted, DNA swabbed and entered in the PNC as a suspect. Then after a few hours (if lucky) they will be released. But their records will be kept.
Over time, more and more innocent people will be entered in the database. Eventually everyone will have their mugshot on record, so it will be completely impossible for the facial recognition system to flag a false positive. Foreign terrorists on their first visit to the UK will be the only people not flagged.