I suspect (mind, suspect!) that approach wouldn't work here in Norway because of the (generally) different geography; in most of the UK, the main problem would

I suspect (mind, suspect!) that approach wouldn't work here in Norway because of the (generally) different geography; in most of the UK, the main problem would be that you got too far away from the transmitter site for proper reception, in which case overlapping like you describe would work fine. Over here, multipath interference is the main problem with FM - signals bouncing off any number of mountainsides before arriving at the receiver, causing a signal transmitted at time T to arrive at [T+t1], [T+t2]...[T+tn] at the receiver, with no way for the receiver to tell them apart as they are all on the same frequency. Garbled mess ensues. Hence, rather than a few huge transmitter sites, we rely on thousands of smaller sites, the smallest of which used to operate at 200mW, IIRC.