For those not really following this then look at :
http://bgp.he.net/AS6871#_graph4Start from the left & work to the right (hover the mouse over the bubbles to get info). Needs javascript for you guys running noscript.
Those are the public routes which Plusnet advertise - or in simple terms this is like a "how to contact us" broadcast for all of Plusnet's IPv4 addresses. I'm using the Hurricane Electric BGP looking glass facility because its got a great graph which shows routing much better than most.
You'll see that Plusnet have only one real public route to the internet - that's Level 3 (AS3356). They also have BTnet (AS2856) but that only provides routing to QWest (AS209). The last "route" you see out of Plusnet is Hurricane Electric (AS6939) which is a private peering link. They will have lots of these private peering links but ultimately they are (IMHO) way too reliant on L3.
Level 3 are one of the original backbone transit providers & their policies are still somewhere in the stone age of the internet in terms of any flexibility - and they cost more than average for very average/poor service IMHO.
The problem with this website is/was that due to Plusnet's reliance on Level 3 transit anyone visiting from Plusnet was having to go to Paris where they'd get hooked up with the website hosts preferred (cheaper) network transit. Kitz' webhost has now decided to fix that problem by doing some private peering (a free connection between networks) with Plusnet.
That indicates a damn good hosting service IMHO - its not their problem that Plusnet are run by BT but they've gone out and done the right thing for their customer. +1 for that.
I guess routing isn't something that even 0.1% of people consider when choosing an ISP