You should declare the foreign self employment income on SA103, in pounds sterling.
So the Self Assessment can include the profits into the tax calculation.
You also complete SA106 (foreign) to declare the tax deducted in Ukraine, so that you can claim a Foreign Tax Credit of up to 100% of the foreign tax deducted.
The guidance notes (SA106) advise "If you’re 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".
National insurance is payable on the foreign profits of the trade, as the profits are taxable in the UK.