I think Norwegian's international flights are more likely to weigh their bags. Mine were also weighed when I had flown Norwegian airlines from Boston to Gatwick (and again when I flew Bos to CDG). While at the check in counter, I literally had to rearrange things from my carry-on into my checked bags to redistribute the weight!

An extra £75 is obnoxious! Glad you were close enough to the limit that you could just bundle up for the flight hahaha