Why are Norwegian classed as a LOCO? Having checked fares recently they appear almost on par with SK on some routes. The other half also interested in flying to Tromso Feb next year to get the Northern Lights "experience". I am somewhat overcome with the fares with both Norwegian and SK fo such delights.
You as a customer are demanding a flight connection on a thin route, which due to the relative lack of demand was not able to sustain itself from the UK.
You will therefore be taking a connecting flight - a service many other low cost airlines do not provide.
You are asking for low cost on a route which is inherently high cost, and I'm afraid neither Norwegian, nor any other airline could deliver that.
I have also looked at prices for LGW-OSL v FR STN-TRF or RYG, and by the time you add on Ryanair's extras + cost of getting to city (even if you don't value your time), there was very little in it. Also, Norwegian are one of the few airlines not to charge you a debit card handling fee.