What's the best / recommended way to mark and manage unused electrical cables
I'm currently thinking I should put both ends into junction boxes, marked with a printed label.


Seems perfectly reasonable to me. 


  And then perhaps mark all areas where it may be visible (I.e., in the loft, under the floorboards and in the consumer box cupboard) with some kind of identifying tape.


Seems like overkill to me. It's pretty obvious that a 10mm2 in a domestic setting is almost certainly intended for either a cooker or a shower.