Hi Nando89,
As you are in receipt of foreign income / foreign capital gains, you meet the criteria needed for the completion of a Self Assessment tax return.
You can register for Self Assessment here:
Check how to register for Self Assessment
In that tax return you would declare your UK savings interest and your foreign interest, converted into pounds sterling.
If you made a capital gain from the conversion of sterling to Euros to deposit into the bond and back to sterling, this would also be declared in the capital gains section of the tax return, even if the gain is below the £3000 annual exempt allowance.
Thank you.