I've have business broadband for the past 4 years, for business purposes I hasten to add. Although my broadband is on my domestic phone line (it as the only on ewhich passed the tests) I have 2 business lines as well. It may be that BT are asking you to confirm your business status as you don't have a business line with them.
Also, with the recent changes in T & C for business broadband, the unlimited download element is no longer the case. Business broadband is now subject to a 'fair usage' policy which may well mean that if you download high levels consistently that BT will bill you. Better check on this yourself.
Apart from that I have no complaints. Good stable line, albeit at 512 as I'm too far from my exchange for anything higher, consistently solid download speeds at or near the max for my service. Customer support is a bit iffy but I've rarely had to use it other than 3 years ago and this week when I needed to get a cease and resume to re-establish my access due to consistent 691 errors.
Hope this helps.
Drummy
Business 500 PnG
Dell 1.9Ghz
256 Mb Ram
XP Home
Intel AnyPoint 3240 USB