With respect, I don't think it is very helpful to tell someone who wants to buy a three bedroom flat in Kentish Town for 500k to beware of estates and unspecified 'dodgy' streets. You simply cannot buy a period flat in a quiet, fragrant, salubrious street in NW5 for that money, not nearly. It will have to be ex LA, or right next to LA. If you live here and have reliable information about the crime rate or disruption on a specific street or an estate, that's one thing, but saying things like beware of living within half a mile of one 'troubled' street is not helpful, especially when that supposed no-go area has enough of a mixed population to support £2m houses, hipster coffee shops, media offices ...
You cannot avoid social housing in Kentish Town; estates make up the fabric of the place, and even in the poshest period streets, there will be some council tenants. The whole point of the area, why young people want to live here, why it's buzzing, is that it's mixed, and hasn't yet become some airless gentrified ghetto. The location of the OP's flat is a case in point; it's an LA block, and bang opposite it is a converted polytechnic with incredibly chic, high end flats worth millions. 1 minute down the road is a wildly popular hipster pub. Around the corner is a TV production company.
OP - as others have said, and I mentioned to your sister, I think your major concern with buying ex LA is the potential Major Works bill.