The main factor affecting your broadband speed is the distance from the telephone exchange to your business premises. Be careful, though: this is the distance of the cabling route, not necessarily the shortest distance from your premises to the exchange.
As a rule of thumb, the closer you are to the exchange, the higher the broadband speed your phone line can support. Although there are many things that can affect your broadband speed, this distance is the most important factor. If you're too far from the exchange, you may not be able to get broadband at all.
This is true of every ADSL broadband provider, not just BT. Every company that can provide you with ADSL broadband has to go from your exchange.