What is the road that connects Upper Street with Liverpool Road?

But you are right there is a sign from the Holloway Road entrance - how long
has it been there?

From Upper Street you can turn directly into Liverpool Road. I haven't
been through that side recently enough to check if there is a 20 Zone
sign there.

By law though, 20 mph zones have to be "self-enforced" through traffic
calming, so if you were stopped there driving down Liverpool Road at
say 27mph, you could probably contest it successfully in court.