You forget that Norwegian already have a very good business out LGW on their Nordic routes, their planned growth at LGW isn't massive for 2013.
Their challenge will be to get people on board, the passenger retention rate will i suspect be high, the product is very good and i understand that 99% of flights will be with WiFi equipped aircraft, there is a certain wow factor on board that you don't get with Easyjet, as for BA a good product spoilt by poor ground handling.
I don't know of another airline let alone a budget airline that gives you a 4 hour cooling of period on its web bookings+ the price you see is what you actually pay plus credit card fee, no admin and £7 a bag is a good deal
I suspect that there will be plenty of people around the Gatwick catchment area ready to try something new, the challenge is in the marketing