An alternative re the TG582N so that it will handle the authentication for you is to assign the external IP to a LAN client and also assign a DMZ to that client. This should ensure all unsolicited traffic is passed to your firewall so VPN or any servers you have will be able to receive incoming requests.
This seemed to work well for me until I had to reboot the modem at which point it lost all of that config!
Re PPPoE authentication I've tried this with no problem with a different modem. The TG582 gets a better sync though hence why I want to see if I can get the bridge working.
If I get time tonight I'll try out one of the methods in my links and let you know what happens.
I'll assume you are talking about the unlimited ADSL product?
The service has been great so far. Traffic falls into the Plusnet Gold and Titanium queues so you shouldn't see much if any deliberate slowdown of your traffic. Can't say I have seen any.
When you say premium I guess you are comparing to Sky/TalkTalk as they are also unlimited?
For me it is worth the extra for access to the great support I have had. On the one occasion I wasn't happy with a reply it got escalated to a senior team member who went out of his way to provide feedback beyond the network problem I had.
Having said that if you use less than 60GB you may be better off with the Plusnet Extra with pro add on package which could be a bit cheaper and has more traffic assigned to Titanium.
For the extra couple of quid though you have the stress free internet experience that unlimited brings