The affidavit of support requires that you include your tax returns for the past three years.
Seeing that you have P60s and P45s, I assume that you had income to report, and therefore need to file a tax return. P60s and P45s are not a substitute for a tax return.
However, there are income limits for filing and if your income is below the income limit for your filing status, you are not required to file a tax return.
Do you think that your income is below the limits, which is why you are saying it's non-taxable?
The IRS at the Embassy has tables showing theseincome limits. I would recommend that you contact them directly and get a copy of the tables. The IRS also has a form letter that explains this, which you can obtain.
If you are below the limits and are therefore not required to file, the Affidavit of Support requires that you write a letter explaining why.
I was the US sponsor for my Brit husband when we did the DCF filing. I'm also a full-time housewife and had no income to report and therefore, I did not file taxes. For our interview, I had the form letter from the IRS (explaining income limits) , the tax tables and the explanation letter from myself. This was sufficient.
But, I am NOT a tax expert; just somebody who recently repatriated and the above is based on my personal circumstances. I strongly urge that you contact the IRS and determine your own filing status.
I hope that this makes sense. Best of luck.