Hi Ella B,
If you are UK tax resident and domicile you are liable on your worldwide income unless there is an exemption with the double taxation treaty.
If claiming the remittance basis, you only declare the income if you actually remit any of it to the UK.
Remittance basis 2022 (HS264)
You therefore need to determine your residence in order to confirm whether the pension is taxable or not under the terms of the treaty with Turkey
Thank you.