You can't really compare the SSDAB with Radio Jackie.
Radio Jackie is really a local station whereas the SSDAB stations are what you would really call Community stations based on Community stations criteria of cost, power, coverage and output. The SSDAB stations are aligned closer to the likes of London community stations Flex, Select and Rinse etc covering smaller areas with powers of 50-200w.

London N SSDAB also uses some 1.2Kw to cover the North London area from its 6 sites.

You also have to bear in mind that a DAB signal has a much wider bandwidth than an analogue signal so therefore needs more power, in the case of London N SSDAB it is 1.5MHz wide compared to radio Jackie's 200KHz.

Also you can't make a direct comparison between SSDAB and local FM, from Radio Jackie's 800w you get a noise free stereo signal in the coverage area for 1 station. From London N's 1.2Kw you get a noise free stereo signal in the coverage area and beyond for 29 radio stations! Try finding room on the band for 29 FM stations covering North London!