Ex plusnet user
Best move i ever made Plusnet were trialling ellacoya and lying through their back teeth about the problems it causes to all traffic.
It works by doing deep packet inspection of all traffic and assigning a priority through the system.Many protocols are easily identified and are assigned a throttling to match or in the case of for example voip no throttling .
Any deep packet inspection requires a loss of speed of the traffic ie ping times increase.Now with the BT/Plusnet model it doesnt matter if you encrypt because if they cant identify it it gets thrown in the max contention stream.
From my own personal experience it was not acceptable to me to go cap in hand to ask why such and such a program doesnt work properly when the reason was they were restricting it.I dont want an isp telling me what i can and cant use my connection for as long as i stay within my contract,so i left and low and behold all the programs i wanted to use suddenly had no problems.
My advice to all is , do not get an isp that uses any packet inspection techniques of the ellacoya type at some point you will find problems, pay a few quid more for an isp that doesnt port block they are available just look for them.You will be pleasantly surprised at how good these smaller isp's are at dealing with problems and invariably give better support than there larger cousins,and many have 1 month contracts so if they do want to change things you can vote with your feet.
I have had 2 years trouble free internet by doing this i hope many more will take the advice get your mac codes and leave them.
Mines the one with Bigger is not always Better !!!! on the back.