No business Domestic but I'm assuming it books in to a TP-earning fare operated by BA under codeshare with CX. Also assuming you mean AKL not AUK:

20 + 120 + 120 = 260 outbound
120 + 120 + 20 = 260 inbound

for a total of 520 TPs. Not enough on its own to get UK Silver and (if considering Eurocheat status) do remember the 4 qualifying BA flights rule. Assuming you have 0 TPs and no qualifying flights already in your year right now (which you don't say in the OP), you need to take the extra flights to tick over a tier. My advice would be to top this trip off with a 'Business Cheapest' EDI-LGW-xxx-LGW-EDI weekend trip and it would net you a further 120 TPs (way more than needed) but more crucially the 4 flights you'd need. I'm sure one of our frequent ex-EDI I class hoppers can recommend the cheapest route (paging Prospero for a start, me thinks )...