If you are resident in the UK and in receipt of income from a non UK employer, you will be taxable in the UK on your overseas employment income.
To declare this income, you would need to complete a self assessment tax return (SA100), declare the overseas income in SA102 and also declare the overseas income in SA106.
You can find all of the double taxation agreement that the UK has, at Tax treaties
Please check the relevant double taxation agreement regarding employment income.
You are likey to find that this overseas income is only taxable in the UK, in which case, you would need to claim a repayment of the tax from the overseas country.