"Please let me know either way"

[ROFTOK]

Basically this "Those days are gone. So, it seems, is the guarantee that British people will respond to emails if they're not interested." means:

Those days are gone as well as the guarantee that British people will respond to emails if they're not interested.

???

 

[emsr2d2]

If that helps you to understand it, then yes, but with "as well as", I'd put a comma after "gone".

 

[jutfrank]

This is the logic:

Those days are gone [and] the guarantee that British people will respond to emails if they're not interested is (also) gone.

Subject
Predicate

Although both propositions can stand independently, the logical implication is that the fact that the guarantee that British people will respond to emails is gone is a consequence of the fact that those days are gone. (I assume that's what emsr2d2 means.)