Military Man,
I did my BTEC Aerospace Engineering Mechanical course many years ago as an apprentice at both Ordinary and Higher levels. This was coupled with plenty practical experience firstly at a general engineering training centre, and then working with the company engineers in the hangars and workshops.
The BTEC course covered all the necessary subjects to give a thorough grounding in aicraft engineering including mathematics, engineering science, workshop processes, industrial law, stores procedures, air legislation, aerodynamics, aircraft structures and systems, theory of flight and jet engine theory and systems.
As to which discipline, avionic or mechanical, will stand you in good stead to become a pilot is a difficult question. I am now a licenced engineer, not a pilot but I know several people who started out as engineers and now fly for various airlines and they come from both disciplines roughly 50:50.
Whatever you learn in your engineering course will be helpful because you can draw on this experience and maybe approach situations or solve problems in different ways or from another perspective.
Good luck whichever path you take.