Guya,
Treaty article 18 allows the contributions to a pension, whether by an employee or an employer, to be tax-exempt in limited circumstances. To make this claim requires the filing of Form 8833, and the amount claimable is limited to the amount allowable in an equivalent US plan. The savings clause (article 1 paragraph 5 (a)) specifically permits this.
However, it has tax consequences down the line at withdrawal time, and it is optional whether one invokes the treaty or not. Even if it is deemed a great idea to deduct the pension contributions, it would require expert advice to determine what an equivalent plan in the US might be (Simple? 401k? Defined Benefit Keogh? Money Purchase Keogh?). Details and particulars of the UK plan would need to be compared with each of these plans to determine the one with the closest fit. These plans have a variety of maximum annual contributions. Many UK plans meet the US definition of discriminatory, and that may affect the maximum annual contribution allowed or allowable. Any excess contributions would be taxable income in the current year, offset by foreign tax credits. Excess employee contributions can also be offset by Foreign Earned Income Exclusion, but employer contributions are specifically exempt from FEIE and can only be offset by foreign tax credits.
Hence, my recommendation that mbmasters seeks some specific US tax advice prior to making such a claim.
IRS Circular 230 Disclosure: To ensure compliance with requirements imposed by the U.S. Internal Revenue Service, we inform you that any tax advice contained in this communication (including any attachments) was not intended or written to be used, and cannot be used, by any taxpayer for the purpose of (1) avoiding tax-related penalties under the U.S. Internal Revenue Code or (2) promoting, marketing or recommending to another party any tax-related matters addressed herein.