Re: Taxation question
 

The US SS administration claws back some of your SS when you retire if you have relatively few years of SS contributions and have non-SS pensions (meaning foreign or US government - there is a huge thread in the US forum on the subject if you want to read more, a LOT more, about it). It is done on a sliding scale, but so long as you can reach 30 years of SS contributions the clawback becomes zero.
Thank you, I had forgotten to mention that. :o

Re: Taxation question
 

Fair enough. As I'll be looking at 30 years here I'll go full steam ahead into contributing to the UK.

My current retirement funding priority is now 401k, HSA, UK Pension, Roth IRA.

Re: Taxation question
 

As was mentioned above you won't have any US SS you get reduced by WEP if your UK state pension is from voluntary NI.

If you are a US resident you probably won't be able to contribute to a UK pension...unless you have some special workplace L-1 visa arrangement. But 401k, HSA and ROTH IRA are all excellent accounts to be using.

FYI I would highly recommend Vanguard for ROTH account.