Sorry, no I meant I will be working self-employed from July 2017 and will be applying for a visa in August 2018.
It depends how the company is set up and whether the tax year runs from April 6th to April 5th, or if it has a 12-month accounting year which is different. I think it depends on if you are registered with HMRC as a specified limited company or as a sole trader. I think the financial year for a sole trader is April to April, and the limited company can be different - but I don't know for sure.
If your financial year runs from April to April, there is no way you can qualify for the spousal visa using self-employment income from July 2017 to August 2018... because you will not have been registered as self-employed for a full financial year by August 2018.
You would only be able to use income from April 2018 to April 2019... and you would need to still be working as self-employed after April 2019, with guarantee that the self-employment income will continue.
Also, even if the financial year of the company is from July to July, if you will no longer have the right to work as self-employed after August 2018, you will also not qualify, because you have to show your self-employment income will continue at at least £18,600 per year in the future - and you can't do that without a valid visa.
So, as far as I can tell, your plan to use self-employment income isn't going to work, because I don't believe you will be able to meet the requirements.