I had a BT engineer in to check my (nominally) 2Mb connection, last week.
According to my understanding of what he told me, an ADSL Max exchange (which ours is) will theoretically give you up to 8Mb - in practice that maximum is actually about 7.5Mb. But you would only get that if you lived right next door to the exchange.
I'm about 3km from the exchange, and he reckoned that I ought to be able to get between 1Mb and 2Mb.
The other factor that can considerably affect your speed is contention - that is, the number of people who are also using up the bandwidth of your BT circuit. You'll probably find that, under the terms of your contract, this could be up to 49 others.
I understand that some ISP's (Talk Talk and Sky for example) fit their own equipment into telephone exchanges, and this can often give you a better broadband speed. But your distance from the exchange is still a significant factor.
You can get a rough idea of the speed of your connection by entering your telephone number at this BT website. click here. But don't rely on just one test - run several at different times/days of the week, and average the results.