My understanding is that if your tax domicile is the UK, you pay UK tax on ALL your earnings. However, if you have already paid tax in another country that has a double taxation treaty with the UK (ie Turkey) you can offset the tax paid in that country against what you pay in the UK.
In simple terms, you pay norma lUK income tax less any tax you have already paid on that income in Turkey.
I'm not an expert so feel free to correct me.
Ian