A constable is not going to be scanning the faces everyone going to Wembley in one night. Even 100 constables looking at faces entering faces going to Wembley is not going to scan everyone and recognise someone they know from a wanted poster (of maybe a couple hundred faces in their head).

The Met have already lied about the scale of false positives[0] by nearly 1000x, and it's not obvious how much better it will get. With the current tech, this rate will get worse as more faces are being looked for. If it's only looking for (I'm guessing) a thousand high-risk targets now and the rate is 1/40, as more and more faces get searched for this problem gets exponentially worse as the risk of feature collisions rise.

Of course, it'll also disproportionately affect ethnic groups who are more represented in this database too, making life for honest members of those groups more difficult than it already is.

The scale is what makes it different. The lack of accountability for the tech and the false confidence it gives police is what makes it different.

[0]: Met's claim was 1/33,000 false positives, actual 1/40 according to this article from last year https://www.bbc.com/news/technology-69055945