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I simply do not know - sorry for that - and may address only your tax related questions.
You may keep asking questions as you wish - and I personally will do my best to address all your concerns.
here are several forms W8 that are used for different occasions - mainly W8BEN and W8ECI.
Abbreviation in the form name Form W8ECI – stands for effectively connected income
(ECI) – thus this form is used if the person or entity is a recipient of U.S. source income which that is (or is deemed to be) effectively connected with the conduct of a trade or business within the United States.
The form Form W8BEN is used for income that is not effectively connected - and that form you need to use.
Because you are a nonresident alien - the administrator of that retirement fund is required to withhold 30% for federal income
tax - and that withholding would fully satisfy your liability
if you do not claim tax treaty benefits.
If you spread distribution over several years - for periodic distribution s you may claim tax treaty benefits - means your distribution would not be taxable in the US and will be taxable in the UK based on your residency.
If you claim tax treaty benefits - that is done on form W8BEN - and there should not be any withholding.
You are not required to claim tax treaty benefits - that is your choice.