In an unscientific poll of one (ie me) the respondent struggled to think of any advertising they have seem from Norwegian in my area of South Manchester. With so few flights, word of mouth isn't going to get round very quickly either. I don't know about the advertising they have round Gatwick's catchments area, but with the much greater volume of flights they operate there they will have become more widely known much quicker.
Ryanair for one seem to be doing quite well from 'conservative and nationalistic' north westerners, despite being quite a recent option for most of their destinations (apart from Dublin). They also advertise a lot......