Not just a problem in the charter game chaps, our lot have lots of lines of work that start very early, involve four sectors in and out of busy hubs and only leave about 15 minutes grace before discretion is required to finish. A lot of crews now, including me, refuse to operator into discretion unless the company have left a decent margin. Often the response is to position another crew on your last sector to operate back whilst you sit down the back. Only 3 sectors now so legal, same time at work so you might as well keep quite and operate it yourself!.
By the way it IS quite legal to dispatch knowing you are going into discretion but again, unless the company has rostered a sensible margin in the first place I figure well why should I put myself out for them.
The CAA need to take a good look at this part of FTL's, they seem to be changing some of the rules regarding early starts and hotels etc. so perhaps they will tighten up some other loopholes while they are about it.