While Media Watch calls the Section Boyz a "UK rap band," they're more accurately described as a grime collective. First of all because "rap band" sounds lame. And also because grime is a different genre of music. But it's close enough.
The point is, the use of their image as a generic placeholder for any "gang" is offensive. In their tweet, Media Watch claims to have alerted the Daily Mail about the photo's origins in November.
If you aren't clear on the implications of using the Section Boyz' photo to illustrate gang activity that has nothing to do with them, here's how the internet responded:
The Daily Mail adjusted their article but, as the Independent notes, "the blurred image of the south London grime collective still shows on Google News for the search: 'Apex gang.'"