You can claim Foreign Earned Income Exclusion based on Physical Presense or Bona Fide Residence tests. The PP test is 330 days. The BFR test requires that you are tax resident in a foreign country.
Depending on how much time you spend in the US and how much in the UK, you may be eligible to file as US resident for the entire year, and claim UK nonresidency under the US/UK treaty. US taxes are generally lower than UK, but not always.
In no way will you be taxed twice, because whenever residency laws do not work in your favour, you are eligible for Foreign Tax Credits that will wipe out the portion of tax in the second country equivilent to the actual tax owed in the first country.
To best utilize foreign tax credits, you must pay any UK balance due for 04/05 on 31 Dec 05 instead of 31 Jan 06.
Send an email if you have any further questions.