Hi,
While the old BT Light User scheme did prevent you from using the line for fixed line broadband (aka ADSL), the replacement scheme BT Basic doesn't have this restriction.
I'd recommend phoning BT and requesting you're moved to BT Basic -
A simple guide to BT Basic - They should send you an application form.
One word of caution though, the BT Basic scheme prevents you from having ISBN on your line - some people have had troubles with signing up to broadband because of this as the BT assistant they spoke to confused ISBN with ADSL broadband. Be assertive, the BT Basic scheme does allow broadband and other customers have successfully signed up.
I hope this helps!