I used an E6B application on a PDA for the PPL written exams. I never managed to find a reference anywhere specifically allowing or prohibiting its use. So instead I asked the CFI who, in turn, asked the CAA. The answer came back that it would be okay to use it for gound exams, but not in the air!
Being the traditional way that pilots do their maths, does not make use of a mechanical slide rule the only legitimate way. If the CAA really thought like that, then other modern innovations would also be excluded from the PPL syllabus. Starter motors, tricycle landing gear, and monoplanes would all be out, and flying would be learned on something that pre-dates "Stick and Rudder".