What I did when I resigned from Sainsburys was :
Wrote my resignation letter, and planned to give it to my department manager (online shopping manager). On the day I took it in, he was not there, so I gave it to the Team Leader, in an envelope marked Private & Confidential. The Team Leader then passed it to HR and within a couple of days, a person from HR came and found me on the shop floor and took me to the office to discuss the resignation and finalise things. I left 4 weeks later on the agreed date.
I considered making 2 copies and taking one directly to HR but there was really no need, so just give it to your manager or the next best thing.
It was such a relief when I knew I had resigned and could count down the days to my last shift!!
AS for code controllers, a good friend of mine is still in that role with Sains. Basically he spends most of his time in the chilled aisles, going along and making sure nothing expired is on the shelf and rotating stock to make sure the older products stay near the front. He also removes items close to expiry to take them for reduction/clearance.
Its quite a responsible job as they seem to have a lot of team meetings to try and reduce waste and stuff.