Such a concept wasn't on their radar, or anywhere near it
So, the IBA didn't like it when their programme contractors were ahead of them with technology!
The origin of pseudo stereo circuits is lost in time. I can believe that such a concept was not on the radar of the IBA which was brought up on mono audio, but I'm surprised that the BBC with their longstanding presence in radio broadcasting had not dabbled about with them.
You might argue the IBA were as passionate about stereo broadcasting as the Beeb. In 1973 they imposed
upon the ILR contractors the requirement to be in stereo. At that time no BBC local radio station was in stereo (and wouldn't start to be until the early 80s) and BBC national radio was restricted to only the main bulk of England.
ILR being in stereo accelerated the Beeb's roll out (some areas were enabled for BBC stereo just days before an ILR station would launch there, a positive effect of competition !) The Beeb themselves were world beaters
using PCM audio for the distribution of the national stations to the transmitters. Compressors and processors were not allowed to be used on FM broadcasts, and so the transmitted quality of UK FM radio in the 70s and 80s exceeded that of any other domestic audio source, and incredibly was of a higher technical standard than anything (FM or digital) we have today.