Use the ADSL availability checker tab located on the home page of thinkbroadband. This will give BT's estimation of your line capability.
Also, note, you are paying for an "up to 8mbs service". Currently with Orange and dependant upon what part of the UK you are located, that can be anywhere between 0mbs up to the high 7mbs level!!!
See the other posts in this section as several folks think that their speed is being capped by Orange. Run speed tests at various time of the day, especially after midnight, to see if it is congestion that is your problem.
http://www.speedtest.bbmax.co.uk