I am sorry, your understanding is wrong.
You get to claim the remittance basis. If you do not claim then you report worldwide income & gains.
A US fund is not eligible for the UK capital gains tax exemption or the 18% CGT rate. Gains are reportable and taxable as income unless you claim the remittance basis & pay the £30,000 charge (individual US stocks are treated more kindly by the UK than funds).
Given that UK rates are 40% & US rates right now are 15% this could be a fairly expensive tax cost.
I read this on the HMRC website, it says that if you choose to be taxed on an arising basis you keep your income and capital gains allowances. Is it the case that foreign mutual funds are not recognized by HMRC and they just categorize capital gains from their sale as income so even if you have a UK CGT allowance you can't use it. It seems that honest US citizen investors are getting caught in the offshore/hedge fund hysteria. I think I'll ask HMRC if US funds sold by firms like Fidelity, T Rowe Price, Vanguard etc that have transparent reporting and issue annual capital gains and dividend information can qualify for capital gains tax treatment
"People entitled to the remittance basis may choose not to claim it. They will then be taxed in the same way as other UK Residents (on the Arising Basis). Tax will become due on worldwide income and gains for that tax year. But the RBC will not apply and they will be entitled to UK personal tax allowances and to the AEA for Capital Gains. They will need to complete a UK Self Assessment return if they have any foreign income or gains for that year.
People entitled to claim the remittance basis are able to decide on a year by year basis whether they want to do so. In one year, a person might opt to pay the RBC and lose their allowances, while in the next year they might choose to pay tax on all of their worldwide income and gains."
Edited with good news.
Well I just found out that my US based mutual funds are classed as distributing funds by HMRC and so are treated just like UK funds. Here's a link to list. It looks like a lot of US retail funds are ok
http://www.hmrc.gov.uk/offshorefunds/dist_fundlist.htm