In the eyes of the law a property is not complete and you do not have to complete on the property, until a COH has been issued and you should not complete until you have one
The only grounds to cancel the contract are based on the actual completion date, the date of the COH and if this has been exceeded by at least 6 months you can then legally cancel your contract and , if you have one, make a claim against the Bnak Guarntee for return of the deposits, costs and interest
In the currebt climate there is a good chance that, if you fail to complete when the COH is available that they will take you to Court and there´s a good chence that you would loose the case in Court and hace to pay costs but they may just walk away with your deposit and then you would have to sue them