After nearly 2 months of slowness and unable to switch to Virgin (I've tried multiple times) I've decided to withhold half of the due amount on my bill until BT fix the issue, they keep sending me letters asking for the money, but won't acknowledge there is a problem. 5MB and a 300ms ping is unacceptable on an infinity connection, I was getting 14Mb after the engineer left he said that was acceptable. My ip profile keeps falling lower and lower and my connection disconnects at least once an hour. The engineer said "there was no problem with the line and there is nothing he can do".
Also my ip profile is 13.2Mb lol so much for 80Mb then. Might think about switching to Be ADSL they recon my line will get 16mb, it'll be faster than this anyway. It's not my computer either, I've tried it on several desktop computers, one server, my housemates laptop, and my friends iPhone. Tried all the usual **bleep** replacing wires etc nothing helps
Personally I wouldn't pay half of your bill because any speed BT offer is based on an up to contract regardless of whether it has been higher in the past you are still getting up to the speed they offered and since they do not "guarantee" a lowest speed (only give a recommendation) I think it would only end up doing more harm than good.
The mods and CLs here will surely be able to give you some help so see what they say.