As a sole trader, freelancer, you would declare the self employment income in SA103.
If you have paid tax on this income in an overseas country, you would also declare the freelance income and tax deducted in SA106. The section "Foreign tax paid on employment, self-employment and other income", advises "if you are claiming Foreign Tax Credit Relief on income included elsewhere in your tax return, fill in the columns below and say in the ‘Any other information’ box, on page TR 7, where on your tax return this income is included.
The country or territory codes are shown in the ‘Foreign notes’. Make sure that the foreign tax being claimed is the ‘minimum’ due under the laws of the foreign country after all deductions, exemptions, reliefs and allowances have been claimed. You can see guidance here: