I discovered that the BT broadband is actually going at almost full speed. The connection is reported as a 288000bps connection, but it seems that OS X is detecting the upstream and not the downstream. I didn't figure this out until trying out the excellent broadband speed test at the ADSL Guide. My downstream results were 444kbps (estimated at approx 480kbps with overheads) and the upstream results were 245kbps. Both values are pretty close to what they should be with a 256up/512down ADSL service.