I agree that Norwegian is likely to be the cheapest carrier. (Landing fees are a lot lower at Gatwick than at Heathrow, so that might partly account for why they're cheaper).
Customer reviews are here:
http://www.airlinequa...m/Forum/norwegian.htm
However, if Heathrow is a more convenient airport for you, you might want to check the fares with Blue 1. On some random dates I checked, in September, Norwegian were charging €117.70 (roughly £102.50), whereas Blue 1 were asking for £137.60. (Those fares are return, including taxes and charges). But there might be some dates where Blue 1 has the cheaper fares:
http://www.blue1.com/en/uk
Chris