In an ISA, the 15% american withholding tax is deducted from your dividend payout before it’s paid and there’s no avoiding it. That’s as low as you can get in an ISA.
However, in a SIPP it can be 0% percent if your broker is a Qualified Intermediary with the IRS. Old school brokers like Hargreaves Lansdown and AJ Bell are 0% but Freetrade (or should it be Gaudi) is 15%.