Generally, I don't believe that you can contribute more to a UK plan than you would be able to contribute to a U.S. plan without being taxed. In the UK it is often better to not claim the treaty, and instead be taxed on the employer pension contributions in the U.S. and then to claim foreign tax credits to avoid U.S. tax.
If you don't claim treaty benefits, the employer contributions are taxable to you in the year of the contributions, and in certain circumstances the earnings within the plan are also taxable to you (these two combined, contributions plus earnings, are known as the vested accrued benefit).
In addition to reporting the pension on Form 8938, it is usually advisable to include it on your FBAR as well.