I worked for the other big UK charter operator and found that they were incapable of sticking to the rostered block times due to creating their own congestion down-route. The three sector days were simply vile with the third sector spent flopped out in the back of a taxi for 2 or 3 hours after a long duty spent dealing with slot delays and disruptive passengers. The stress for me over the summer just built up due to the long randomly rostered duties and missed ATC slots, with insufficient downtime on off days to restore a safe level of alertness.
I got out before I parked my car in a ditch on the long drive back to my home base.