Your speed is controlled by automatic equipment in the telephone exchange (the "DLM"). BT have very little control over this; they can reset it to it initial state, that's about it. If there is a "punishment" setting to penalise users who have gone beyond their initial broadband contract period then this is a very well kept secret.
Here is a basic guide to getting help from the community members done by CL Keith Please read through the link posted http://forumhelp.dyndns.info/speed/first_steps.html This link also may help Particularly the part posted by Forum member RogerB http://forumhelp.dyndns.info/speed/speed_drop.html
once you have posted the information asked for then the community members can help you more
This is a customer to customer self help forum the only BT presence here are the forum moderators