I am not aware of all the intricacies of the various schemes, but AJ Bell does have one advantage (and this might be common across most providers... I am not sure and stopped investigating this at AJ when researching for the son).
This bit might be a good thing to look at if you're into high dividend etfs etc...
https://www.ajbell.co.uk/our-service...tional-dealingA W-8BEN form is not required for US investments held within a SIPP as the IRS recognises our SIPP as a qualifying pension scheme and all qualifying US dividends and interest are automatically paid to you free of any withholding tax.