Remember you are never going to get the full speed on a speedtest due to overheads, background programs using bandwidth, heavy local traffic, heavy traffic on the remote server (it's being OcUKed

), distance from exchange, quality of line, distance between router and master socket, etc etc etc
Further to the above, as with all BB providers it is an 'Up To' 24Mbps service, the provider is in no way obliged to provide the full speed if it is not possible in your case. Though if you are getting that sort of speed maximum all the time it may be worth seeing if you can get a cheaper 8Mbps service as it should provide a comparable speed