Nobody knows exactly how much the tariffs on items from UK to the US will be, so I would expect the GSP to not be applied to US sales for months.
From August 29th orders coming into the US will be subject to an import fee, the de minis exemption will be gone. The de minimis exemption had allowed goods valued at $800 or less to enter the US without paying any tariffs.
For six months, a temporary flat tariff (ranging from $80 to $200, depending on the item's origin) may apply to postal shipments, so I can't see many sales being made to the US even if you set your own international postage price and method.
