Languages
For the 16+ project, for teaching, Hoskyns chose to make use of Elliott's Symbolic Input Routine (SIR) assembly language, one of the languages that was available on their Elliott 903 computer. This was the main assembly language for the 903 range.
SIR and the Elliott 903 — Computer Conservation Society
After the project was taken over by ICL, and they began to develop their material for 16- students, the team developed their Computer in Schools Instructional Language (CESIL). This was a simplified low-level assembly language developed for educational use.
Here's a page from the Revised Book 1 16- teachers guide that explains their thought process with CESIL.
More detail to follow on the languages. However, Ian Dunmore has already done an excellent job of explaining CESIL on his website, and I advise you to read this.