The ABS light comes on with the other ignition lights array & should go out some seconds after start up. If it stays on it indicates a fault somewhere in the ABS system (could be one of the wheel sensors, the control unit or the valve block assembly) which can be time consuming & expensive to fix. Unfortunately it is also an MoT fail item (as is the light not coming on at start up). So, you have a big problem. If you bought from a dealer you may have a chance of getting something done.