Cazzz's answer is misleading.
While such things as the distance from your exchange place limitations upon your broadband service (which will be common to all providers using that exchange) different ISPs can still provide different speeds. That's because other ISPs don't 'rent lines'; they rent exchange capacity. Budget operators have 'contention ratios' of up to 50 to 1. (That's the maximum number of subscribers who they'll connect to a single point on the exchange). Better ISPs have contention ratios of around 20 to 1. ISPs which serve mainly businesses (such as Claranet, which I use) have a 7 to 1 ratio.
In some exchanges, where 'local loop unbundling' has been enabled, third party ISPs have their own equipment installed within BT exchanges (bypassing the need to rent capacity from BT).
To see if any third party ISP has installed their equipment into your local exchange, enter your phone number here:
http://www.samknows.c...dband/exchange_search
When the name of your exchange comes up, click on it and look under 'LLU Operator Presence'. If any companies are shown, they should be added to your list of possible providers. If not, google to find out the contention ratios of any ISPs which you might be considering.
Chris