The dividends that you receive on stocks you hold through your personal pension would still be subject to tax. In paragraph #3 that you sited, when they refer to "pension scheme", they are referring to a company that offers a pension to its employees where contributions from the employee are not required. The zero percent tax does not apply to stocks you purchase through your own personal pension funds, so the 15% tax would still apply.
If Hargreaves Lansdown will not handle the W-8BEN forms, then they under US law
they would withhold a full 30% tax on the dividends. They should still not withhold any tax on the sale of the shares, but only on dividends paid. If they do withhold at this higher rate of 30%, the only recourse you have to recoup the difference of 15% is by filing a tax return each year with the US. If you prefer not to deal with this paperwork, then you would just end up forfeiting the extra taxes which were withheld.
I think the best case scenario for you would be to find another broker who will handle the W-8BEN for you, as then they should withhold the appropriate tax of 15% and you would not have to be burdened with filing a US tax return to obtain a refund.
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