If you are working overseas and pay tax there, you can usually claim what is called a Foreign Tax Credit. The UK has an agreement with nearly every country in the world, so when you pay tax overseas you can enter it in your Tax Return and the UK tax due can be reduced by the overseas tax paid. However, it is actually more complex than this. According to HMRC guidance notes, you should in fact work out the tax that should have been paid there according to the foreign tax rules. If you have overpaid foreign tax, then you should register in that country and claim back the overpaid tax and only then enter the amount in your UK Tax Return.