Joining up several different comment upthread,
IIMU this :
QUOTE
Modern Railways back in the 1970s or 80s had a report on coal MGR trains with 2x20 at the head and 1x20 mid train.
The mid train loco was described as radio controlled, but it wasn't explained what this actually meant
There was a photo of a 20 mid train with the report.
I never saw it referred to again, so may have been an abandoned experiment
UNQUOTE
was BR trying out one makers product - possibly the US GE Harris product, well known and in use at the time in US, and probably the one that a.n.other mentioned upthread.
Knowing BR, they probably either did not want to pay for the product or product licence, rather than a technical or operational test failure, so went their own way with RTC developing what was fitted to 20058+20087, which probably took them quite a while, or the business needs changed.
I have wondered if\how\where\when the remote control 47(s) fit in - the underlying concept of remote controlling a mid train helper or rear end banker by radio from a lead loco cab is not that different to remote controlling any loco by radio from a trackside control console.