If you are only getting 2 megs then you either have not been upgraded to 8 megs at the exchange, you live a long way from it, or you line is crap.
I've got a 12 meter extension on my system from the master skt to the computer and download quite happily @ 7.5 megs from a good site.
Make sure you have the extension cable plugged directly into the master skt (if you have a phone there use a splitter and an ADSL micro filter for the phone and then another micro filter for the computer)
Also try running a speed test, just google it and choose one (bt is a good one) it will tell you what the speed of the line is set to at the exchange as well as what you can actually get.