As this is ADSL you may find that the speed is limited by two factors,
A) the distance you are from an exchange
and
B) The quality of your in home telephony wiring.
For A, you cant do squat about it, for B) you can.
To check, move (if possible) your ADSL router right to your BT master socket, it looks like this:-
Then switch on your router and see what it connects / downloads at, if its (significantly) faster at downloading you may have internal wiring issues on your telephony cable.
To check how far from the exchange you are and what speeds you should expect - goto
http://www.samknows.com/broadband/checker2.php and put in your post code, then when the results show up click the Locality tab and it will show you your straight line distance from the exchange. Anything more than 2km and your speeds will drastically reduce.
You can also go here,
http://212.23.23.177/calc.htm, and put in the details for your router to check what you should be getting based on the noise on your line.