The range of the RNI FM service was helped massively by the fact that there were very few stations on the FM band back then. These days there are no completely clear frequencies on FM around the south east of England. Back in the days of RNI there were no FM broadcast stations above 98 MHZ in the UK and there were still plenty of gaps between those. There was just the radio telephone type comms networks which spoilt DX reception between 98 and 102 MHZ in some areas. I used to be able to receive the bigger London FM pirates back in the early 1980's at my location in North essex using nothing more than a telescopic antenna on a sensitive Grundig portable radio.Thats around 40 miles distance and most of those early London pirates ran well under 100 watts back then.
I now struggle to hear FM stations running 2 to 4 KW that are only 30 miles away due to co channel interference from other stations. Maybe the FM band will start to empty again slowly as more stations move to online distribution only giving oppertunities for DX reception of remaining FM stations (and pirates !) again.