Hi,
US government bonds, sometimes known as T-bills or treasury bills are generally taxed as income rather than capital gains. Where they are disposed of before maturity, any difference between the price at which they were issued at and the price received on their disposal, will generate a gain or a loss.
On a foreign investment the income is the difference between the purchase and redemption price after each has been converted to sterling on the day the transactions took place, so includes any foreign exchange gains. Losses cannot be deducted.
UK gains from deeply discounted securities are entered at SA101 box 3 and foreign gains from deeply discounted securities at box 41 of SA106 and then claim Foreign Tax Credit Relief at box 2.
You can see guidance here:
SAIM3010 - Deeply discounted securities
Thank you.