I'm on the PlusNet Premier package which is £21.99 per month. The 30:1 contention ratio applies to inside the plusnet network, not the BT one.
As far as stability goes Ive never really had any trouble with plusnet (ive been connected to them for about a year now). Speeds are fine as well. Thier customer support is excellent in my opinion also.
I would go for the premier package over the plus package as the plus package have idle timeouts once they impliment it (you get disconnected after 30mins if the connection is not active, but you can reconnect straight away) Take a look at adslguide.org forums for more info on that as im not 100% on it.
As for unlimited bandwidth, there is a catch which is the sustainable usage policy. You can find more info about it here:
http://www.plus.net/support/adsl/sus...ge_guide.shtml
There is also bandwidth capping on usenet when you download over 15gb a month.
If you need to know anything else, either reply here or pm me, im happy to help.
EDIT: found some info on how they manage different bandwidth types
Gold – Web browsing, email, PlusTalk
Silver - Gaming, VPN, network services, other VoIP services, external text Usenet servers, FTP, SSH, Instant Messaging, IRC, other TCP/UDP/other traffic, P2P/Usenet for PAYG/Lite accounts
Bronze - P2P for all accounts bar PAYG/Lite, external binary Usenet, all binary Usenet for Broadband Plus