Work out if we can do it in hours.Can't.Apply discretion .Go.
...
"Are not into discretion untill we are half way home so we are NOT dispatching into discretion"
Wrong, wrong and wrong again. This is totally illegal, and dangerous. You cannot take off from Tenerife without exercising your discretion THEN. In such a case, the company should either send another crew with the aircraft, or position a slip crew down-route, and nightstop the first crew there.
As Sky9 says, if you can't do it within the terms of CAP 371, don't do it. Have you thought exactly
who would be the one to be hung out to dry should anything go wrong? I can just see an Operations Manager saying "...that's not what we rostered - they
said they weren't fatigued, and elected to continue..."
I say again - it's NOT legal -
DON'T DO IT.