With driving lessons costing a fortune these days, it is a good idea to go with a cheap instructor. However an instructor who charges less money for a driving lesson should not really be compromising on the quality of the lesson. I think your driving instructor is completely out of order after reading your message and I think you should complain about him straight away.
If you haven't signed any papers, then don't worry as you will be able to sue him if he forges your signature. However you need to understand that if you do not have any contract stating that you've booked for block lessons, then it will be difficult for you to get your money back because he will deny the fact that you've already paid him in advance.
If you have a mobile phone which can record voice (or a similar gadget), try tricking him into a conversation where he admits taking the money from you so that you can use this evidence against him. You can also film him and his wife sitting at the back when you are driving and show them talking on the phone with a spy camera if possible.
If you don't mind losing the money (depending how much he owes you), then don't bother as you will need to gather solid evidence first and probably go to court to sue him but you can still lodge a complaint against him through the Driving Standards Agency
and see where that leads you; they might help you recover your money or suggest better ways (by the way, DSA takes complaints very seriously).