At one point BT were throwing heavy users off by either making them pay for the excess bandwith or allowing users to migrate away from the service even if you were still in the 12 month contract period.
Since "MAX" came in, and users shifted onto telephone bill payment - it seems as though they will simply warn you of overusage and if you do it again then you'll be charged
BTYahoo!
1Mb stats Atten: 52.5 SNR 18.5
MaxADSL Atten: 53.0 SNR 6.0 (448Kb up / 2080Kb down)