Oh really? I thought the destroyers need only be present in the sea zone to neutralize the subs’ abilities.

I believe that’s what it says in the manual 😕

Then again, I haven’t familiarized myself with any errata.

This IS in the manual and has no mention in the Errata (though it might be in the faq portion).

On defense, any units that are allied together defend together.  But on Offence ONLY the attacking power participates.  This is how it has always been.  So on the US turn, only us units are involved in the attack, the british destroyer does not participate and thus cannot allow the bomber to detect the subs.