LLU = Local Loop Un-bundling, this is the concept introduced by ofcom to force BT to allow other ISP's access to the copper from the exchange to your home - this is done by the ISP installing their own equipment in the exchange and BT forward on all ISP activities to that equipment.
This will not have an effect on the bandwidth you are seeing as the bandwidth is determined by the line conditions between yourself and the exchange. Typical factors that will affect the bandwidth will be the distance from the exchange - this is electrical length rather than physical distance as the cable may not take a "as the crow flies" path to the exchange.
Typical reasons for a reduction in bandwidth can be found in your home - the use of unshielded extension cables etc can have a big impact as can running the telecoms cable from the DSL modem near AC cables. For optimum speed you should plug your DSL modem into the BT Master socket and avoid any power cables etc running near as these can introduce noise onto the line.