OK. I spoke with three RE teachers, one of whom has taught in a school with a significant number of muslim pupils. We have some but as a proportion of the entire cohort, relatively few.I'm not clear as to whether this was a school policy, an instruction from the RE teacher you were covering for or something from elsewhere.
They informed me that the written guidance is for sensitivity, however if a lesson was being observed and images of the prophet or Allah were used as a resource or drawn/downloaded by a pupil then the lesson would almost certainly fail observation.
In our school we operate guidance which states that it is not necessary to depict either the prophet or Allah in order to master the syllabus content and therefore issuing explicit instructions not to draw them is appropriate because it further enables the pupils to later recall this aspect of Islam.
I have to admit that I hadn't thought this through before. There are some great Islamic schools in the UK and there are those where Ofsted has had to close them with immediate effect. I have had the fortune of working with some great educational professionals who also happened to be muslim. I think the DfE are aware of the issues found in a minority of these schools and have begun to deal with them effectively.