I too am London based, but for me I've been back and forth to NZ many times in the past few years, primarily because my father in NZ had terminal cancer, so I never lacked a reason to use RUs, although since he passed away I am dropping down status too, and BA has become the programme I have more loyalty too.

I have used RUs LHR-LAX with little problem, the key is booking flights with more availability in the premium cabins. It typically isn't too hard to get a Y-PE upgrade. PE-BP is more difficult, but far from impossible. One option if you want to make sure you can burn them is to book in Y to LAX and PE back, and use the two RUs to upgrade Y-BP outbound, or just PE outbound and BP back. Bear in mind around half of those on NZ LHR-LAX are NOT NZ FFs, but UA/VS/LH and your chances as a Gold of burning two RUs are quite good. However, if you don't really want to go to California then...

As far as Airpoints are concerned I find the best value use for them is First Class *A awards. If you want to search for availability, sign up to be an ANA Mileage Club member (it is free) and then online you can search general availability. It used to be possible to search on Expertflyer, but no longer.

I find LH and LX to and from Japan/South Korea to generally have availability (commonly in business, less so in first), and flying LH in First from FRA is well worth it given the First Class Terminal and overall ground service (at both ends of the flight). Of course you are just short of enough for that in Airbucks, but if you have the time and money, flying NZ to and from LAX in Y/PE would top up your account just enough to splurge on a one way in First to Tokyo or Seoul. If Africa is of interest, that's an option too.