Foreign saving interest conversion

I have got a foreign saving interest of USD 1000 and exchanged it into GBP 780. I found it later that according to the monthly exchange rate, the interest is to be calculated as GBP 800. Could I report GBP 780 as the saving income or GBP 800?

Hi,
It will be the £800 unless you actually withdrew the interest and exchanged it for sterling in the tax year.
Thank you.