One of the challenges with a computing curriculum is that it is very quickly out of date. Technology moves fast, and as an example, the AI has a brief mention on current UK syllabi. The jobs that current students will take probably don't have names yet, as they have yet to be invented. I've seen this all through my career as a computing science teacher and lecturer. Syllabi content has to be consulted upon, agreed and ratified. A particular syllabus will be in use for ten or more years. Currently I have to teach students about network hubs, disk thrashing, optical media, defragmentation and so on, because a question could potentially appear in a future exam paper. There's currently no date for new GCSE or A level revisions, only consultations, so they are at least a few years away.
Between the first editions of the books and the late 1970s, technology had evolved. In the late 70s, microcomputers such as the Commodore PET and the RM 380Z were starting to appear in schools. They had VDUs that provided instantaneous output based on input. Program code could be input, run, edited and run again in a matter of seconds. This was a problem for ICL and the ICL-CES project because computing was changing quickly, from mainly the batch processing model to the interactive local model. ICL manufactured mainframe and mini computers. A whole raft of manufacturers were releasing or about to release many different models of affordable micro computers.
ICL didn't operate in this space and that was a problem because a large part of their ICL-CES programming element was based around ICL technology. Write your code on pre-printed sheets, send them off to an operator to produce punch cards or paper tape, which would then be input into an ICL minicomputer or mainframe, and then wait up to a week or so for the results. This was the modus operandi at the start of the ICL-CES program but it was seemingly becoming old-fashioned and out of touch towards the end. Yes, teletypes and interactive terminals existed, but students didn't always have access when they wanted to these computing services.
Some students liked CESIL, but others didn't. Jeff Minter, the famous games programmer didn't like it, and was frustrated with the old school method of programming. Minter, Jeff, and Patrick Minter. A History of Llamasoft, s3.amazonaws.com/lsshop/AHistoryofLlamasoft.pdf.
Schools had spent a lot of money on teaching materials where a large proportion of the course involved programming in a (now) old-fashioned way. So of course for a few years they continued teaching using it. Students were learning CESIL when they had an actual Commodore PET sitting in the corner of the room. Some teachers with access to a microcomputer created a CESIL translator (see last comment).
Yes the concepts and principles that were part of the ICL-CES programme were still valuable knowledge, but without in an class translator, teenagers would have viewed this programming execution model as old fashioned, frustrating and not as fun or interesting as directly interacting with a microcomputer.
I can understand why ICL sold the CES project in 1983. ICL itself was struggling with its own financial issues, partly due to the influx of cheaper microcomputers and increased competition from companies such as IBM (leading to a takeover by STC in 1984). Their product line did not support delivery of a computer studies specification that wasn't a perfect fit with the incoming microcomputers. The learning materials would have also needed to have been revised. Acorn was in a strong position with the success of the BBC Micro. Best to just sell the project on. Note that Acorn didn't republish any of the computer studies materials. They published two book (Computers, Information and You), a relatively small ~125 page book, and later, "The Information Age". and then followed a strategy of producing educational software and training, rather than text books.