Hi user888,
Bank interest falls under income tax legislation, as does NS&I bonds, so are taxed as income, not capital gains.
Up to 5 April 2023, the criteria for declaring capital gains in a tax return, was £49200 (4 times the annual exempt allowance) disposal value.
For 2023 to 2024 onwards, this was changed to £50000.00.
If a tax return is required for any other reason and the assets disposed of exceed £50000 in value, then the gains are required to be reported in a Self Assessment tax return.
Please have a look at the guidance notes for SA101 (additonal information) as NS&I income bonds should be declared here:
Additional information notes
Thank you.