Hi cross_entropy,
You are taxed on your world-wide income in the UK, so you are required to delcare the interest in your overseas savings accounts.
You will need to convert to pounds sterling to declare in the tax return.
Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal.
For your convenience, there are exchange rates at:
Exchange rates from HMRC in CSV and XML format
You can utilise an exchange rate from elsewhere.
Thank you.