This is not the issue. There will be footage of no end of dippers chucking pint pots over the edge. The problem will be linking the footage of any individual pot thrown and showing it was that particular one that caused the injury. There is little doubt in my mind they have found the guy throwing the pot by linking the time stamp to the time of the injury.
If you could tell from that alone that the pot had coins in it, there would have been an arrest. So my guess is they know who they think threw this particular pint, but they are making sure they have the evidence that clearly connects the pot being thrown with the girl being hit. It is possible to do this by use of multiple camera angles and time stamps, but it is easy to see that unless you have watertight evidence that the defendant would be able to raise a reasonable doubt in court that his particular missile caused that particular injury even though it is clear what he threw.
Let’s face it, when he’s arrested this scrote isn’t going to hold his hands up and say “it’s a fair cop, guv.”
They are taking time because they want to nail this bastard.