BA Silver/One World Sapphire here. Most of my travel is done on BA as I'm based about 30 min drive from LHR.

However my fiancee and I are flying from Copenhagen to Christchurch and back in December in J (D class fare), and I'd like to maximise by some distance the biggest amount of money I've ever spent on a cash fare.

Is there a Star Alliance programme I could credit the flights to and get an equivalent to One World Sapphire in one go? I believe I'd get KrisFlyer/Star Alliance Silver, both of which don't appear to have any real benefits.

Or am I best-off crediting to Alaska Airlines which I believe would give me a fairly decent amount of miles which can be used on loads of different airlines, including BA, but I would effectively give up the Elite Miles (tier point equivalents)?

Most of my travel for 2020 is booked other than one long haul that we haven't made our mind up on yet and I'm carrier-agnostic currently.

Cheers,
Jack