These prices are flat rate for any speed (the 'based on 2Mbps' in that article applies to the right hand column about the achievable speed, the prices as most people have understood it are the same for any speed).
PlusNet, like most ISPs, thanks to Capacity Based Charging from BT Wholesale are now saying that you don't pay for speed any more you pay for capacity. Basically you can have the fastest speed your line can support for the basic price.
With PlusNet that means you can (when it's available) in theory get 8Mbps for £14.99 a month under the Lite scheme. That's a 1GB cap of course and you then can pre-pay £1.50 per GB to add to it (£1.75 per GB PAYG).
For Premier customers you pay for a "fair use" kind of cap (you have to exceed it 3 months in a row though, and any downloads between 1am and 8am don't count).
For me, I use on average 16GB a month and pay £39.99 for 2Mbps Premier. This announcement means I can pay £21.99 a month for 30GB (and I can exceed that in one month without a problem so long as I reduce my use the following months).
Not only that, if it turns out my line can do 8Mbps when it's available... I get the speed increase for free!
Fantastic
Quitting NTL for PlusNet was definitely the best choice
NTL are living in the dark ages with their pay-by-speed charges and draconian caps
Of course there will always be some people who will complain about caps, though these aren't strict caps on the Premier products. Those who (like the "bad boys" on PlusNet) use huge amounts (400GB a month!), have plenty of other ISPs to choose from who will cater for them, and then there are business/teleworker packages too.