You can't plan into discretion but there are rules which are designed to cover seasonal operating plan changeovers. Only a proportion of the flights have to be doable during a particular window which explains why it is completely legal to have a flight time greater than the block which would lead to discretion.
As to believing it is your discretion, it is devolved authority from the NPH and it would take a brave man given an order from the big man not to 'help out'. Not saying some wouldn't and I am sure I will have a load of outraged comments now about moral high ground but the reality is the pilot group have shown themselves to be a bunch of pussys that will just roll over and take whatever they are given. In that climate megalomania flourishes.