Update with a decisive data point: I ran a real (non-cancelled) sandbox purchase through the minimal repro project. The resulting Transaction.storefrontCountryCode = "GBR" and Transaction.currency = "GBP" — correct. But Storefront.current, called immediately after and again 3 seconds later in the same process, still returns countryCode "USA", id "143444". This shows StoreKit has the correct storefront information available (it used it to process the transaction correctly) but Storefront.current and the product catalog query are not returning it. This looks like an internal inconsistency between StoreKit's transaction-processing path and its storefront-reporting API, not an account or device configuration issue.

Update: built a minimal, dependency-free StoreKit 2 project (same bundle ID, same team, zero third-party code) to isolate this from my main app. Storefront.current itself resolves incorrectly: countryCode: "USA", id: "143444", with locale fixed to es_MX@currency=usd — despite the Apple ID's Media & Purchases region being set to UK. This confirms the storefront resolution failure happens at the StoreKit/OS layer, not in app code.