Meal Reliefs are such a can of worms.

Bearing in mind
ALL Delivery offices are different and
ALL Managers have their own attitudes to MR. Personally I see nothing wrong with not taking a MR during my day finishing 40 mins early and booking those 40 mins as MR. Some Units/Managers are fine with this. The only problem is if you come back "Early" and a Manager says, "You've still got 40 mins work left", hopefully you know your Regular Managers views.
I know The CWU want everyone to take MR during the day, but sitting on a wall somewhere in the Arse End of Nowhere in the wind and rain, eating soggy sandwiches is not my idea of fun.

I know some Offices stop at a certain time and all disappear to the canteen......... In my experience most Posties just want to get out of the door, get their mail delivered and knock off as soon as possible.
Law....Surely your Manager is not so dim that he doesn't realise if you stop for a 40 min break during your day...you are going to finish 40 mins later?
IF you decide to Cut Off first talk to your Rep and make sure you follow the correct procedure or you'll drop yourself in the shite. Ironically I believe if you follow that procedure you'll be asked to take a 20 min MR so the Manager can count the number of Delivery Points
you aren't visiting so he can work out your call rate. So that's 20 mins he's lost already!

Good Luck.