I haven't got good news I'm afraid, but don't panic. Seeing as you didn't have a valid MOT certificate when you were stopped by the police then you've committed the offences of driving a vehicle without the correct documetation and failure to produce. The police will refer the case to the magistrate's court and you'll receive a summons. If you accept that you were in the wrong and don't plan to submit a defence, then you can plead guilty by post so you don't have to appear at court. You'll probably receive a fine of a couple of hundred quid and get points (3?) on your licence. You can negotiate with the court and pay the fine in installments that you can afford.