I've read through the US/UK DTA, and some info on the SSA website (EN-05-10199.pdf, and EN-05-10045.pdf).
It appears that I was wrong, the US/UK DTA is considerably different than the US/NZ DTA in terms of how a public pension is determined. The taxation provisions are still quite similar, but the US SS calculation is dependent on the years of substantial service in US, the years of credit in UK, etc. I've not seen anything showing that there will be a deduction on the US SS as based on other pension payment. Frustratingly, the calculation basis is not defined. At least, I've not been able to find the appropriate definition.
I will declare myself ignorant in regards to the details of the SS determination for US/UK, please disregard my previous comments about the public pension capping as it is clear that the process is considerably different than what I have become familiar with for US/NZ.
Regards
