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you have to prove enough income of 100%  of the poverty guideline for your household with a I-134 at the interview. And later with AOS with 125%
 
https://www.federalregister.gov/documents/2018/01/18/2018-00814/annual-update-of-the-hhs-poverty-guidelines

For a K-1 visa, neither USCIS nor NVC will look at your financial situation. Only the CO looks at it when the I-134 is presented at the embassy stage.
Later, when you file for AOS, USCIS will look at it with the I-864 that gets submitted.
 
Just to be clear....are you currently self employed or are you an employee of the company? The reason I ask is because if you're an employee, then you're current income can be shown via pay stubs and an employment letter. If you're self-employed, then your current income will be demonstrated via your tax returns.
The most recent year's tax return is required either way, but the significance of it will differ depending on your situation.

It is possible to do the interview in the country of residence or the country of nationality.
NVC will assign the location after the petition is approved.  If the assign Sysdney and you want it to be London you simply contact the consulate and request the change.  It takes typically about two weeks to get the file moved.  No specific form to do this

I wouldn't be so bold as to predict acceleration, but this trends graph seems to indicate that CSC processing time for K-1s has at least leveled off.  Unfortunately, it is still 4 times what it was in Jan 2017.