Talk to solicitor / conveyancer.
On exchange of contracts, a legally binding commitment existes on both sides to complete the deal. Either an agreed date for completion will have been agreed, OR a back-stop date will have been agreed between the parties (beyond which a party that does not complete is in breach of contract.
If one party fails to complete to the date / back-stop agreed, it is in breach of contract and you can claim damages from the other side for your costs (so, for example, the extra expenses incurred in reselling).
So whilst it isn't in the bag for you 100% yet, the number of deals that go wrong after exchange of contracts is tiny (because it becomes very costly for the defaulting side).
I wouldn't worry too much but do now talk to your advisor, armed with that info.