Low cost long haul doesn't work. Air Asia tried it first out of STN, then LGW. Oasis tried it and went bust. Even Hong Kong airlines with their all business class was a failure. BA have given up offering first class on a 744 from LHR-BKK due to the lack of yields, and are moving to a 3-class 772. BKK just doesn't attract the high paying passenger like HKG or SIN.
There's no way BA, Virgin, AA and United will let Norwegian be a success on the most lucrative route in the world. Remember AA starting their US service out of STN when EOS started flying? As soon as EOS went under, AA pulled out of STN.
Unless they can fill the front cabin with people paying £5k a ticket, there's no money in it.