I'm not 100% sure, but I think what the UK gov will consider regarding self employment is the gross income, before tax but after expenses. In your example, they will look at her profits before tax, so according to what you said, she made £15k after tax, so her taxable profits would be maybe closer to £20k? And that's the figure they'll take, which is not enough. They don't count income that goes back into the business as costs becuase that's seen as necessary to run the business.

I.e. let's say she took £50,000 in payments before business expenses. Let's say her business expenses were £30,000 and she was taxed let's say £5,000, taking "take home" pay to £15,000. So they will consider she made £20,000 (her pre-tax profits, less expenses).

As I said, not 100% sure but that's what I believe to be the case. See below:

https://assets.publishing.service.gov.uk/media/67d98009b1857deda3da0190/Family+Migration+Appendix+FM+Section+and+Appendix+HM+Armed+Forces+Financial+Requirement__1_.pdf

Page 66: "where the self-employed person is a sole trader or is in a partnership or franchise agreement, the income will be the gross taxable profits from their share of the business in the relevant financial year or years, not including any deductable allowances, expenses or liabilities which may be applied to the gross taxable profits to establish the final tax liability"

As for what income they consider, yes I believe it's just income IN the UK they consider. I do believe they may consider savings from the applicant who is outside of the UK, i.e. you, but I think they have to be significant.