In the SA102, you would show the actual pay and tax as shown on your P60. The element of the pay that is not taxable in the UK, would then be showns on SA101,
Self Assessment: additional information (SA101)
Page Ai2 in box 12 " Foreign earnings not taxable in the UK". For online tax returns, you would select Other tax reliefs and deductions, when tailoring your return and enter the amount of income not taxable in the UK. This entry will give a credit in the tax calculation, for the income not taxable in the UK.