The law, in my opinion, is in this case an ass!
It may be an ass. but it's still the law!
'Valuable consideratiuon' is the bit you need to worry about if you do unpaid instruction. The student has still given his 'valuable consideration' to the flight, even if you don't see any of it, so it's still aerial work.
I had intentions of keeping up the instructing when iIstarted in an airline. I only seriously got back into it after 8 years! It helps to be with a charter airline during a quiet winter