I am not able to comment on the current working practices at easyJet. However, having had experience of a computer roster system which changes aircraft and crews, I proffer the following:
It is no defence of any working practise merely to say it is within CAP371, is the law and is therefore satisfactory to all concerned. I have never heard an airline pilot say that 'because it's CAP 371 it's OK'
Some (and I means some) changes are inevitable in a high rotation operation. But to build these in to the system is to create yet more pathogens in a safety critical system.
I recall having a new First Officer per sector on a three sector day. The day had its aggravations (don't they all) but having to repeatedly build up my team from scratch made it a darn sight harder than it need be. It is much better to form up a team for the day andiron out all those little communication glitches that inevitably occur when folk are getting to know each other.
Fatigue again comes into it. Add aircraft and crew changes, in a duty which has many of its own challenges, to our system of organisation and you quite obviously make the day more tiring.
For my money the best roster system must be drawn up with the co-operation and understanding of the crews that have to work it.
Imposing working practices on safety professionals will always be a hit and miss affair with inevitable friction.