Personal opinion counts for very little in this context. What matters is the law and how the insurance company or, in the worst case, a court interprets it. As usual, UK law is written in such a way as to allow for the maximum flexibility in interpretation and, specifically, ANO Article 136 prohibits anyone from "acting as a pilot" without being in possession of an appropriate licence, unless undergoing flying training. An insurer might argue that if a person influences the flight path of an aircraft by manipulating the flying controls, they might be said to "acting as a pilot". If that person does not hold an appropriate licence and is not under instruction it might be further argued that the flight was being conducted other than in accordance with the ANO and, therefore, that the insurance was invalidated.
Of course, this will all remain as pointless conjecture unless and until the issue is tested in a court of law.