It's very depressing. Here's what the councils have to say about it:
https://www.islington.gov.uk/~/media/sharepoint-lists/public-records/communications/information/adviceandinformation/20182019/20180501workingwiththehomelessinstroudgreenroad1.pdf
Thanks detritus -- it's the same sign as the ones on Moray Rd but the markings on the cat look slightly different to the one we saw last night. I've emailed the owner anyway just in case.