Non-UK residents will need to show proof of income to be eligible for a mortgage. That means, with the proper documentation, you can ask for a loan, even if you're self-employed.
You may have to meet a few requirements with some lenders, for instance, some will require foreign nationals seeking a UK mortgage to have an annual income of at least £75,000. Besides that, you'll also likely be asked for your most recent income tax assessment.
To make your life easier, if you are a self-employed non-resident, then hiring an expert mortgage advisor from When The Bank Says No will help you get together all the necessary paperwork in the proper formats to give you a better chance of securing a mortgage with the minimum of delay.