To defend a FF program (which I never thought I would ever do!) Norwegian is actually amazingly generous OAK-LGW but only if you buy Premium Flex.
It looks from rough math that you earn about 100GBP a leg that you can use just like cash towards a future booking. This is huge compared to United etc.
A discount Premium fare OSL-LGW was about 7% of the 100GBP credit, so they really want the high yield fare but bizzarely give back a big chunk of it.
Economy I am sure you get almost nothing unless perhaps you buy flex, which would be very silly to do for the product.
Enjoy it while it lasts before IAG arrive!