Hi E,
Please refer to:
https://www.gov.uk/hmrc-internal-manuals/savings-and-investment-manual/saim3080.
This states the loss is set against your income charged to Income Tax for the year.
Also, as a Deeply Discounted Security (DDS) normally a US Treasury bond, would be subject to Income Tax on maturity for UK taxation purposes. However, if redeemed or sold before maturity would be subject to Capital Gains Tax, therefore, capital gains (CG) liability or loss may arise.
CG54602 - Deep discount securities: CGT adjustment
CG54602 - Deep discount securities: CGT adjustment
Therefore, any gain would be entered onto shares and securities area of CG schedule.
If double taxation applies any relief would be claimed on the Foreign schedule, CG area.
SAIM3080 - Deeply discounted securities: taxation: losses
Thank you.