Or maybe the culprits aren't humans at all. Leaving aside the number of foxes reported killed, I would suggest that foxes could well be the culprits in many of these reported cat killings, not least because foxes do routinely decapitate prey and leave the bodies behind when they kill surplus to their immediate food requirements.
I'd be interested to know how many people have been convicted of killing cats like this. Where cats in a local area are killed, they are frequently poisoned, and the blame is frequently laid at the feet of a local person fed up with other people's cats *advertiser censored* all over his garden.
The most readily found conviction is of Franky Mills who did porridge for shooting cats, most of which survived.
http://ukcriminallawblog.com/franky-mills-gets-2-years-for-cat-killing-spree/
Clearly decapitation does not come into the picture in either shootings or poisonings.
I'd also have to question the ability of one or a small number of individuals to carry out so many attacks across so wide an area.
I'd suggest that in urban and suburban environments such as Croydon and other cities where attacks are reported there are now many foxes living in close proximity to humans, and that foxes and cats do have an overlap in terms of prey.
That and the seemingly very small number of convictions suggests to me that the issue is the urban fox rather than an army of psychos.