Re the Droitwich LF transmitter bandwidth, BBC Engineering #89 of 1972 January included a longish article entitled: “Reduction of Interference by Reduction of Modulation Bandwidth”, page 23ff.
Therein it was noted that the Droitwich transmitter was 1 dB down at 5 kHz. I have attached the pertinent pages from the article:
Also, here is a frequency response chart from a 1935 paper on the Droitwich station. This could well have changed over time with transmitter and/or aerial upgrades and replacements.
The restriction in bandwidth due to aerial Q was not immutable, though. The late Allouis LF transmitter was designed to have a ±10 kHz bandwidth – then the CCIR number for AM broadcasting - as delineated in this Wireless World item:
The above-mentioned BBC Engineering article also mentioned compression as well as bandwidth restriction.
A subsequent brief item in BBC Engineering #97 of 1974, page 27 described the FL4/55 low-pass audio filter intended for use in the programme feed to MF transmitters. This provided a steep cutoff above 5 kHz, with a 3 dB lift at 5 kHz.
The BBC EID sheet for the FL4/55 was dated 1973 June, so I’d assume that this is about when it was introduced into service. Prior to the use of this filter, MF broadcast bandwidths were essentially limited by the GPO landlines, typically 10 kHz and lower, although the lines to Brookmans Park had been at 15 kHz since the 1940s. I think that at some later time, the MF bandwidth cutoff was moved out to 6 kHz, but I don’t have a solid reference for this.
The editorial in BBC Engineering #97 was “A New Plan for L.F./M.F. Broadcasting”. Mentioned in it were bandwidth restriction, audio processing, and a common channelling plan for ITU Regions 1 and 3, with 8 kHz suggested! At the time, Region 3 used 10 kHz channelling.
Thus, we could mark 1973-74 as the “turnover” point, after which bandwidth restriction and compression became the norm.
Cheers,