There are some interesting details here...
One is the "is not taxed" proviso. One should not expect a free lunch. TANSTAAFL.
Another is the public pension and SS. I've not read through the details on the US/UK DTA sufficiently to confirm it is the same as the US/NZ DTA, although they use quite similar language and wording. For the US/NZ DTA, the result is that you get the highest of the two as a payout, and not a cent more. If the UK public pension payout ends up being higher than the US SS payout, then, you would not get the US SS payout. If the UK public pension payout is lower than the US SS payout, then you can claim the difference between the two from SS.
Any private pension payout will not be used to reduce the US SS payout, regardless of whether the private pension is sourced in US, UK, or other countries.
