Bicycle Tag of bike

So south side of Balls Pond until South Gate (after which all of) is Hackney? North is Islington?

Yes, only the little bit north of Ball's Pond Road between Kingsland Passage and the A10 is in Hackney. From the Southgate Road/Mildmay Park junction westwards, BPR is in LB Islington.

You can see borough boundaries as the thick light purple lines on Streetmap.

(it's still in Hackney, though)

The place in the photograph certainly is.