I gather from the fares you quoted you are looking at an MGLOB29 ticket. I just did a quick look at the route and came up with about 32,000 miles, excluding the PPT-AKL segment which would put it over the 34,000 mile maximum.

When I mentioned the savings from originating in CAI, I was assuming you were going to 4 continent Oneworld Explorer ticket. The MGLOB34 low season fare, ex-LHR is £1669, ex-CAI it is £1200, and ex-BRU it is €2900 (£2000). Thus, you will save about £330 by flying from LHR vs. BRU but an additional £470 (£800 total) by originating from CAI -- possibly worth the effort.

QF used to fly their own metal from PPT but discontinued that in 2000 in favor of a PH codeshare that could be booked on a Global Explorer ticket. However, I believe that PH dropped the PPT route and that QF now codeshares with TN which is not a GE eligible carrier.