As with most metrics, the US uses a different scale. There's RON, and MON, and now we have AKI, which basically is (RON+MON)/2. They're tested using different methodologies, so it's not just a simple numerical conversion.
You can't directly compare US 87 grade gas to UK fuels. If it's 87 AKI, that's roughly the equivalent of 92-93 octane here; 87 RON is the equivalent of 97-98 octane super unleaded.
Mid grade gas (US) is about the same as UK regular unleaded.
All now further complicated by the practice of blending varying amounts of ethanol.
Cars sourced locally should be set up to use a specific fuel (check your handbook) available in that local market. Imports are a different kettle of fish.
