Erin, there are two sorts of ADSL broadband that most people can get in the UK via a copper phone line.
The first thing you have to know is that in the beginning of broadband, the only company that supplied it was BT, and other companies complained about BT's monopoly, so Ofcom forced BT to sell its product at wholesale rates to those who wished to re-badge and re-sell it.
So, the first sort is of broadband was either a BT product (like BT Broadband) or a re-badged BT product which the ISP buys from BT Wholesale, and re-badges with their own name. You can get these products at any BT exchange because they are BT products. Because both BT and your ISP (e.g. Virgin) are making a profit, you are less likely to get as good a deal as you would get from BT themselves, because you have a 'middle man'.
The other sort of broadband is what us known as 'unbundled' or 'LLU'. LLU came into being because it was ruled that re-selling BT products again gave BT a sort of monopoly, and BT were forced to allow equipment from other suppliers into their exchanges. LLU doesn't use BT's exchange equipment, and it does not use BT's optical fibres for distribution - it uses the ISP's own network.
The snag is that some BT exchanges don't have enough physical space to accomodate the equipment owned by other ISP's, so not all LLU providers will be available at all exchanges. If you look up your exchange using the link that I provided, you will see a list at the bottom right showing all the LLU services that are available at your exchange. Note that you can search for your exchange by phone number, and it's not safe to assume that you will be connected to the nearest exchange

Virgin do both a re-badged BT and an LLU product, and if they have an 'LLU presence' at your exchange, they may well allow you to transfer to their LLU product without any financial penalties. But you won't know unless you ask
