Without a current MOT certificate, you will be unable to drive your vehicle lawfully or renew your road tax. The penalty for not having an up to date MOT certificate could be a fine. Also if the vehicle is involved in an accident you may be asked to produce your MOT certificate. An insurance claim could be affected by the absence of your MOT, especially in the case of injury.
Recent computerisation of the MOT testing system by DVSA means police and mobile camera units can now check remotely to see if your vehicle has a current MOT.