Does not look good Im afraid.
You will be in breach of contract and the builder can (a) forfeit your deposit and (b) sue you for damages ie his losses. So if he cant sell to someone else for the same price as you were paying he can sue you for the difference plus additional legal costs,estate agent costs interest on his borrowings...he would have to prove these losses but once he can you are liable. In recent years they did not bother as they invariably sold at a higher price but now its a different market.
Also a third option is to sue you for specific performance..i.e bring you to court and get a court order forcing you to complete..this is pointless with people who cannot afford to buy but in your case you can you simply have changed your mind.
Your best option if you really want to pull out is to negotiate with the builder,ascertain whether he is likely to be able to re-sell easily and then make him an offer,he wont necessarily want to drag you through the courts either even though he may be prepared to do so.