Its not even compnany that stops you doing it.
Its EU-OPS.
Unless you have holiday or 3 days off(not required) in a row you can only really do a bit before or after a shift if there is room for you to use the time without incuring a penalty in your rest period.
Most of us that enjoy instructing have banged our heads up against this. Even if the company is supportive very quickly it becomes a bit of a nightmare with FTL's.
Some guys try to get round it by just not logging anything which is of course illegal.
And others just ignore the fact that they are busting FTL's and don;t tell company.
I only know one person third hand that got caught instructing on thier days off. And they were grounded for 2 months, 1 month to get thier 900hours in limits and 1 month to teach them a lesson. They were very lucky to keep thier job as they had enough leave to cover 6 weeks and they had an OPC anyway at the end of the 2 months.