I'm also trying to find the answer to this question.
I've opted to have x% deducted from my salary each month and matched by a similar sized (up to a maximum) contribution by the employer into a pension plan.
I've found this in the double tax treaty which I interpret as being able to exclude both contributions from my taxable income:
Happy to hear if I have misunderstood!
(a) Where a citizen of the United States who is a resident of the United Kingdom exercises an employment in the United Kingdom the income from which is taxable in the United Kingdom and is borne by an employer who is a resident of the United Kingdom or by a permanent establishment situated in the United Kingdom, and the individual is a member or beneficiary of, or participant in, a pension scheme established in the United Kingdom,
(i) contributions paid by or on behalf of that individual to the pension scheme during the period that he exercises the employment in the United Kingdom, and that are attributable to the employment, shall be deductible (or excludable) in computing his taxable income in the United States; and
(ii) any benefits accrued under the pension scheme, or contributions made to the pension scheme by or on behalf of the individual's employer, during that period, and that are attributable to the employment, shall not be treated as part of the employee's taxable income in computing his taxable income in the United States.
This paragraph shall apply only to the extent that the contributions or benefits qualify for tax relief in the United Kingdom.