Actually, it's a bit more complicated, as both of you are 100% right, for different reasons. The US govt uses a 1099 matching system, so the numbers on your return must match what was reported. Thus you are right. Yet, you only *owe* tax on your 1/2, so Guya's right too.
For US accounts on a US tax return:
1) Report the full amount on your US tax return. This is because the US govt links the numbers from the 1099 to your US tax return, and the numbers must match.
2) If the amount is substantial, you report a nominee distribution to the second person. That's a pain in the neck, but worth it for large sums. Net tax effect is tax on 1/2 the income to you, and 1/2 the income to the second person.
For UK accounts on a US tax return:
3) Just report your 1/2, as it wasn't reported to the US govt on a 1099.
For US accounts on a UK tax return:
4) Just report your 1/2.
For UK accounts on a UK tax return:
5) Just report your 1/2. You can add an explanatory note stating that the account is a joint account which earned £XX, of which 1/2 was yours.