It depends on the size of your fleet, the 3 fighter scamble isnt always that big a difference.
Attacking norway + denmark also requires him to trade both or lose the NO as well.
Also it gives you a staging ground for going to leninggrad which causes him to lose even more NO income. He can ignore Normandy as you cant do much from that location, its only 2 ipcs not a big deal.
Norway is 8ipcs lost.
So its not 20-30pu its more like 6-12pu, as the AA guns from london are basically free.