Just to add a couple of thoughts to Terry's excellent reply, when you are on a copper wire connection at a long range from the exchange then it's important to check a few things, to make sure there are no basic faults in your set up.
Have a look at the following:
1 Check that your BT Home Hub is plugged into the BT master socket (don't try and use a socket that is an "extension" ....it should be the master socket.
2 Perform a speed test using your customary wifi connection and then immediately perform another speed test with wi fi disabled and using an Ethernet cable twixt Hub and computer.... is there any significant difference in speeds between wifi and Ethernet? (you may need to perform the test a number of times to rule out as far as possible any temporary/momentary differences.
3 Next try plugging in your Home Hub into the test socket of the BT master socket (the test socket on modern BT equipment has a moulded horizontal line running across the face of the socket and the face plate is removed by unscrewing carefully the two securing screws and moving (again very carefully) the face plate to reveal the test socket directly underneath. check speeds again using both wi fi and Ethernet.... if the speeds achieved directly via the BT test socket are significantly greater than when plugged into the face plate socket then this indicates a problem with the telephone wiring in your home.
4 Check that your ADSL filters are working OK, try new replacements if in doubt, and make sure that all BT sockets, extensions as well, throughout your home all have an ADSL filter installed correctly ... Note any "splitters" used should be downstream of the ADSL filters, the ADSL filters being plugged directly into the BT wall sockets.
5 If you use Wireless range extenders these can reduce your speeds and sometimes the reduction is significant... if you need to extend your range then its usually best to use an Ethernet cable instead, although Homeplugs that use the houses internal 230v wiring can be an effective solution.
6 Wifi signals can be attenuated by the presence of walls, doors, ceilings, twixt your BT Home Hub and your computer, and also can be affected by machinery, metal filing cabinets and other barriers to the transmission of a good signal, Interference can also arise through the use of DECT phones, baby alarms and other equipment working on the same or similar frequencies.
Lastly some folks hide away their Home Hub (router) often placing it on the floor behind or even underneath furniture etc and that's a sure way of decreasing the effectiveness of the signal... it should be out in the open, and just as with the placing and orientation of a portable transistor radio, some trial and error on its exact location and orientation may be required with speed checks on wi fi to identify the best position.
When you are getting slow speeds because of your distance from the exchange then its important to not lose too much speed once the signal gets to your home and hopefully these simple and very basic tips may help...
Sorry if they seem too simple and basic for your particular situation but maybe they will help someone else who comes across this problem and is looking for a solution via a search engine.