The speeds we quote on our website for FTTP, eg 115/20 or 1000/115 relate to the line speed and not the actual throughput you can expect to have when downloading files. Our Gigabit FAQ covers some of the reasons behind this.
As FTTP is a shared service - other properties in your neighbourhood will be sharing the same fibre cables and equipment etc. Our suppliers specify a minimum speed for traffic through their network. This is the minimum speed you can expect to achieve during the busiest 3 hours in the day (typically the evenings). BT also state that "In reality services typically operate closer to the purchased speed for the majority of the day."
With that said, our aim is that our network is never the bottleneck and we keep a close eye for any congestion in our suppliers' networks, and we expect customers to be able to use the full throughput of there line as and when needed.