There isn't a direct 1:1 relationship between the two, and as such it would be impossible to do as you suggest.
Each Plusnet account could have a large number of mailboxes associated with it, so there would't be an easy way to know which one to log you into.
Likewise, you could have a mailbox set up for somebody who doesn't have permission to make changes to your account, thus being automatically logged into the PN account pages wouldn't be secure.
I agree that the entire PN portal is a pain - I've been using it for a number of accounts since around 2005, and I still find it a pain
