Gerald has sort of replied to mrkfm's question, but I'll do it too...
Islington can reject it because they'd rather have a smaller number of homes that are more affordable. Other Boroughs may have decided that they need as many homes as possible, even if they're not so affordable. Bear in mind as well that market rents in Islington will be higher than in most places, so the Council may consider it particularly un-palatable there.
My personal view is that Islington should, like many others, done their homework to find a middle ground where homes were 'quite' affordable and there are 'quite a lot' being built but they have made their choice and only time will really tell whether it's right or wrong.