Hi,
Any amount of overseas interest must be declared in a Self Assessment tax return. The fact that the capital is affected by currency fluctuations is irrelevant. The interest is the amount of Turkish Lira paid on the capital in the account and when converted to pounds sterling, it should be declared in a tax return as foreign interest.
The following guidance advises that as currency, other than pounds sterling, is considered an asset and its disposal can give rise to Capital Gains Tax or allowable losses.
CG78300 - Foreign currency: introduction
Thank you.