Again: read the first clause. And be aware that headings, preambles, etc have no legal effect in statutory interpretation. There's nothing in the clause (or surrounding legislation) restricting its power of preemptive violence to _prevent_ a crime --citizens arrest requires that a crime has been committed (past tense)-- to police officers.
Fundamental precept of Western cultures: the citizen gives over the power of taking the law into their own hands, and the power of violence to achieve an end, to the State. The State has the only authority --and a responsibility-- to commit violence in the pursuit of social ends, on behalf of all its citizens, and typically only via specially authorised officers.
To be clear: vigilantes are forbidden, vigilantism is a crime. In every Western jurisdiction I'm aware of.
Except in the UK since 1967.
It IS possible that there is some other separate legislation somewhere which specifically restricts this section. But neither I nor 2 British lawyers I've pointed this out to have been able to find anything. It appears to stand as-is.
It's not just an aberration; it's an inversion of the law. It's hilarious. (And useful to know. Although I dare say a horrified correction will be slapped over it shortly after anyone actually USES it in a court of law and brings it to official legal attention.)