Could be just a simple case of outdated records. I was in a similar position many years ago when I first tried to subscribe to the old BT Vision service, (computer said no, the line speed wasn't fast enough, although all checks said it actually was). Best ring BT and explain the situation, they will clarify things.
Hi I have just been told that although I have speeds upto 21mb you need 30+mb to get the entertainment channels. Is this right or was I being fobbed off?
You were being fobbed off by untrained overseas technical support reading from scripts and then making it up when the script has no answer.
BT will sell the TV Entertainment package to customers who have BT Infinity broadband along with Fibre Multicast availability. They will sell BT Infinity broadband to customers where the line is capable of supporting FTTC-VDSL or FTTP at speeds of at least 15Mbps. That ties in with my own measurements where a multicast HD channel uses approx 7.5Mbps (these will probably be variable bitrate) and the box allows you to view/watch two HD channels at the same time. If you want multiroom with two boxes then a faster minimum speed is required.
Is your current 21Mbps on Infinity or standard broadband? The entertainment channels are not available over standard broadband (ADSL) with Copper Multicast. Only a few sports channels are delivered that way, and normally only if Infinity isn't an option for you.