Contention ratio is one answer that may fit. - too many people trying to do the same thing as you want to do on a not fat enough pipe.
Think of a water main - if your area has a 4" main and 10 of you turn on the tap - lots of water, if you have a 1" main and 10 of you turn on the tap - a trickle.
You need to check whether you exchange has been LLU if it has then it will tell you which ISPs have equipment in your exchange. Also check the
However they will all use the same copper cable from the exchange to your house. BT is responsible for all the copper whoever you use as your ISP.
Since the law was changed, in the last year, the ISPs have to give you an estimated speed. So check on your exchange suppliers - also check carefully for contention ratios - the cheaper packages are often 50:1 but a good package could be as low as 16:1 but unfortunately there are often reluctant to mention these figures.