Originally Posted by
Antarius
It's a different connection, but it isn't airside. You have to re clear security.
It's airside in context that you don't need to exit through Immigration and Customs.. They just require re-clearing through security, which has been SOP at LHR for as long as I've flown through there. Just different from US "airside", and definitely different than connecting through the US to another foreign destination which requires clearing ICE even without any intent on stepping on US soil otherwise.