Was this a Barclays Growthbuilder plan?
According to the notes here, the profit is subject to Capital Gains Tax not Income Tax. I seem to recall having one or two of these or similar over the years and I always had to check or have the client check with the issuing bank.
I hope this helps but let me know if you have any further questions.
If you have previously invested in a Barclays Growthbuilder and income tax was deducted from the "profit" on maturity, I would check with the bank itself as to their undetrstanding of the tax treatment givne that tax is no longer deducted from interest. Tax would never be deducted from capital gains.
You only need to declare capital gains if the gains for the tax year are in excess of the annual CGT exemption which is currently £11,100 or the disposal proceeds of all such capital transactions for the tax year are in excess of £44,400 (4 x the annual CGT exemption) regardless of the level of the gians overall.
The capital gains pages can be found here.