I have been looking for the legal reasons
Try this: the requirement is to fly the approach for which you have been cleared,
using the equipment specified on the plate for guidance. In this case ILS. If you
do not have the ILS guidance there in front of you it isn't 'legal'.
If you
do have the ILS for guidance then you may use any other equipment you like as an 'aid to situational awareness'
but not as a substitute for the primary guidance.