ANPR in the UK works on a very heavily regulated target: UK vehicle licence plates. The size, font and spacing of the letters on these plates is legislated for, and non-standard plates are hunted down by the police.
By contrast, a human face is a very much more difficult target to recognise. Generally speaking, most systems up to now (notable examples being purported terrorist-spotting systems at airports) have suffered from such huge false positive rates that they were useless. I would strongly suspect that the system that the police here are employing would suffer from similar levels of false positives.
Repeatedly arresting a person on the basis of unproven technology would, if done frequently enough, count as harassment and it is for a court of law to decide (based on previous precedent) what frequency of false arrest would constitute such harassment. UK courts have not looked particularly kindly on police conducting trawls like this in the past, so I therefore suspect an ulterior motive for this surveillance.
I think this is not intended to catch criminals, but rather to frighten them out of the area.
UK criminals are typically rather stupid individuals in the main, and are also already used to pervasive CCTV and how to evade it (the standard chav uniform of baseball cap and hooded top is intended to conceal the face from overhead CCTV), and are well versed in what powers police have as well. As a result, I rather think that most criminals here would just go to the festival anyway, and trust police incompetence and the inherent uselessness of the technology to keep them unmolested.