I will try but you may not be convinced. Certainly there's a broad evidence base justifying primary prevention approaches, from the WHO [0] and perhaps at least the references in [1], and in the BMJ [2]. It feels slightly pedantic to have to then prove that taking these types of intervention _away_, or at least reducing them, would therefore lead to a rise in the rate of violence, but it'd be valid to demand that. Perhaps this report [3] from a more local perspective might help paint that picture better.
Many millions of pounds have been taken out of the council's budget. This has impacted youth, mental health, and alcohol/drug services. Changes at the national level have also effected the benefits landscape for many families in poverty. There is quite plainly less help and stability for communities most plagued by violence.
My wife works in and around child and adolescent mental health services and she and her colleagues clearly perceive the link, but I also hope it's evident from a systems thinking perspective: there is a pipeline from childhood poverty and trauma into more chaotic and violent behaviours. Anything that interrupts that pipeline is good, and taking that away is probably bad. Sure, it's an option to wait at the end, increase stop and search and just arrest everybody but that's not a particularly great public health outcome either, and perhaps there are lessons from America in diverting funds from everywhere else just to prop up police departments.
Much obliged if you would provide a citation for this claimed causation.