I assume they were always doing it. Now they're doing it above the table so that they can comb through the dataset for petty reasons that don't justify the expense of parallel construction.

Police have been using it in London for several years, mostly at protests, but also in other 'test' deployments. They are currently required (it seems) to place signage around the catchment area to notify the public that facial recognition is in progress. Private companies may well have been using it for longer in their shops without having to notify anyone. Sainsbury's, for instance, quickly attracted suspicion that they were doing trials of it at certain locations.