I've been reading this thread with interest as I had never heard before that the legendary Radio London had ever even changed frequency at all. It was always just given as being 266 Metres. I guess that they were all only very small changes and as all radio's back then were rotary manual tuning so people would just twiddle the control slightly to optimise reception regardless. Being 'off channel' only mattered if there was a background station to beat with and produce the dreaded horrible heterodyne note. Family members I have who remember listening to them were all close to the Essex coast so the signal was so strong they would have probably not noticed any het in the background, even at night.
The only other offshore station around Britain I have read about that used an Off channel frequency much later was the short lived Radio Mi Amigo from their own ship after they parted ways with Radio Caroline around 1979 and operated off the Dutch coast before running aground ?
With Medium wave now being around 80% empty (where I am In in N Essex)you could operate off channel on loads of frequencies if you wanted to but there is little point and few receivers would be able to tune into you exactly with only fixed channel jumps apart from communications receivers. I've still got one old school manual rotary tuning radio that is in full working order - a Grundig Yacht Boy 1100 made around 1977. Happy days.