Hi,
A Deeply Discounted Security (DDS), such as a US Treasury Bond (T- bill) would normally be subject to UK income tax on maturity, like a chargeable event gain, unless it is redeemed or sold before maturity, then the transaction may be subject to capital gains tax (CGT).
Please therefore refer to
SAIM3010 - Deeply discounted securities: introduction onwards &
CG54600 - Deep discount securities: introduction onwards as you will need to determine whether the charge will fall under income tax or CGT in the first instance.
You may wish to seek independent financial advice if you are unsure which type of bonds you have.
Thankyou.