I am currently with fast.co.uk, unfortunately I'm on a small rural exchange (20cn only STEASTK) and get some slow speeds at peak times, usually between 7pm and 11pm broadly speaking, I had the same issue before xmas and decided to give 'max premium' a go as understand this has lower contention ratio and priority traffic over BT's network, however this doesn't seem to have helped and now I'm paying £45 a month and I'm frequently getting between 1 and 5mbit during evenings, probably about 2-2.5 overall (I sync at 8Mbit), I average about 25gb per month up&down.
I'm not sure if the problem is exchange congestion or an ISP issue, I can no longer get the tap3 test on BT wholesale's speedtester to work but before xmas I was able to get 6.5mbit+ using the BT speedtest login a number of times where as using the ISP connection I would get average of 2mbit, fast say there is no problem with their capacity and BT via fast are saying no problem either so I'm a bit annoyed at spending £45 a month to get a 'value level' type service, I would rather get a better service and pay in this region but failing this I will just go to a cheapo ISP if BT is the problem as there is nothing to gain by paying more if the exchange capacity is fubar!!.
Don't know if you can advise if you would likely be able to improve things?, if you can I will request a MAC as I just seem to go round in circles with fast, they are generally helpful in answering but in the end nothing changes and the issue never goes away. Out of interest would an ISP be able to see if there was exchange level congestion or are they just relying on BT passing this information on (when it suits).