“…don't know why Islington should be unhappy, plenty of wealthy people and places there.”
As garaman has explained, like many London boroughs, Islington has extremes at both ends of the affluence table. I know it well because I went to school there. What you see on the telly are the leafy squares containing large town houses that politicians inhabit. Indeed these are very nice. But these make up only a small part of the borough. There are other parts of Islington which display quite a fair level of poverty and deprivation. There are lots of large council estates, particularly between Essex Road and Upper Street, in Finsbury Park, Barnsbury and off Holloway Road. Even the area towards the City of London (City Road, towards Bunhill) is not particularly affluent. Many of these areas contain large town houses but most of them have been converted into flats, which are not particularly attractive.
Hackney is similar as is Lewisham and Camden. In fact, most of the Inner London boroughs display a wide diversity of affluence (and presumably, with that, goes a wide diversity of happiness).