I don't have a definite answer to the question. However, I did extensive interviews with three long time recording engineers for Decca for my Decca book I wrote in 2013-4.
John Dunkerley shared with me some of his data sheets (Studio plan and Electrical Plan) which I included in my book. They were for his famous recording of the Arnold Dances on Lyrita (SRCS 109) with the London Philharmonic with the composer conducting. Decca did all of the stereo recordings for the Lyrita label. By the time of the Holst Planets recording in April 1971, just about all of the classical orchestral recordings used the three microphone Decca tree in the middle with two outrigger mikes to the left and right, all at the front of the orchestra. There were typically several highlight mikes placed strategically around the orchestra covering specific instruments.
What distinguished Decca recordings from most of the other labels was that all of the mikes (in the case of the Arnold Dances there were 17 mikes in total) were mixed to two channels and recorded directly to two channel tape recorders all in real time. They normally did not record to multitrack and then remix after the recording. So everything was set up optimally before the session began. The Decca Tree was developed by Decca engineer Roy Wallace in the mid '50s at the beginning of Decca stereo recordings. The famed Kenneth Wilkinson modified the tree and that became the standard, particularly after Roy Wallace stepped down from his role as an engineer. Jimmy Lock was the main recording engineer for the Planets recording at Royce Hall on the UCLA campus. I have one picture of the LA Philharmonic at Royce Hall with Mehta conducting which shows the Decca Tree, the two outrigger mikes and several highlight mikes, similar to the studio plan from Dunkerley. However, the picture doesn't indicate the year and date of the session nor the piece that is being recorded.
So my guess is that the Planets recording is probably simply miked (although that could mean 15 or 20 mikes in total), and mixed down to two tracks in real time, and not recorded in multitrack.
Larry