Welcome to FlyerTalk
xmassheep
I am only going by what you wrote but it appears that you are after shoulder room not legroom.
For 777-300ER flights to/from LHR you are out of luck. The Economy seats are narrow, similar to 737 and A320 Economy seats, slightly narrower IMO. I would as a singe traveller choose a window or aisle AB or JK seat in rows 57-59 as the seating is 2-4-2 see
seat map
For the 767-300ER flights to/from RAR I suggest the best bet to get a free seat next to you is the centre aisle seats, as aisles and window seats are assigned first and the seating is 2-3-2 see
seat map
You can join BMI and have it loaded into your booking however NZ does not usually give *A points for low economy fares but only some to Airpoints accounts.
I am going by the fact that you booked a cheap economy flight, after 1 June 2011.
Now Airpoints earning is based on "journey" so you will in fact get APD credit for London to Auckland, either 45, 55 or 65 for the 3 lowest categories.
Airpoints is an awful program but try not to let this ruin your trip to NZ.