UK tax exempt pension schemes will also be entitled to zero, provided that the dividends in question are not derived from the carrying on of a business, directly or indirectly, by the pension scheme and that more than 50% of the beneficiaries, members or participants of the scheme are individuals who are residents of either the UK or the US (Article 10(3)(b) and Article 23(2)(e)).
In summary therefore, in order to obtain zero a person must be a UK resident, must satisfy the conditions at Article 10(3)(a) or (b), must satisfy one of the relevant tests in the limitation on benefits article and must satisfy all the other specified conditions for obtaining the benefit.
In USA - UK case, USA does not charge UK and UK does not charge USA its a bilateral agreement. SIPP has nothing to do with employers. you can(and should) open a SIPP yourself.