Some evidence that the B7G base was preferred for industrial applications in the UK was provided by this item in Wireless World (WW) 1950 February. This announced that the Scientific Instrument Manufacturers Association (SIMA) had issued a list of preferred valve types, on the octal base for “standard” valves and on the B7G base for miniature valves.
At the time, the noval base was probably too new in the UK to have been considered, although one might expect that some noval types were added to the list later on. I think it is a safe bet that the EF91 was included in the initial list as the preferred high-slope miniature pentode.
In the mid-1950s, the dominance of the noval base for domestic equipment was noted by Baxandall in his WW 1955 January article on a gramophone and microphone preamplifier, thus:
“The equipment as described uses Noval-based miniature valves; but certain other valves may be employed if desired, and the slight changes in circuit values then necessary are indicated below Fig. 1. The Noval type appears to be becoming established as the preferred series in British commercial practice, combining excellent electrical characteristics with conveniently small size and satisfactorily robust construction.”
Baxandall chose the EF80 for the output stage of that preamplifier, for which high-slope was desirable because it also served as an R-C feedback type low-pass and high-pass filter stage. Circuit details were also provided for the alternative use of the EF91 and the SP61. Thus one might say that the EF80 was primary here because by the mid-1950s, it had become the “go to” valve for consumer equipment where a high-slope pentode was required, including in applications where the differences between the EF91 and EF80 were probably not material.
More generally, in UK practice (and probably in European practice too), one of the VHF high-slope pentodes was the choice for MF and HF applications where a sharp cutoff pentode was required. In part this was because the UK/European valve range did not include HF pentodes that were directly comparable to the American 6AU6 and 6BH6. Both of the latter saw some use in UK equipment, particularly where Brimar was the valve supplier, and elsewhere, with the 6AU6 sometimes under its EF94 designation. But generally, one could day that where the American designers used a 6AU6 for HF applications, the UK designers would have used an EF91 or EF80.
Cheers,