I have been told by cabin crew before that it is ok to move seats during the flight but have to stay in the assigned seat during take off and landing (though that has always been on the E170/E190, would doubt this also applies on the big birds where weight distribution seems less of an issue).

But regardless of the reasons formoving back into his seat, it appears common decency to say something when moving back and to ask about putting away other passengers things, even if it appears to be rubbish. I probably would have challenged the guy in a polite/friendly way. Then again I am missing the British passive-aggressive-self-depreciating gene.