I have written on the subject of standard contracts HERE and HERE.
The most valuable standard contract most often encountered in ordinary life is the one used by solicitors for the sale and purchase of a house. It is the work of the conveyancing committee of the Law Society.
It is the work of many people over many years. It embodies the experience gleaned from many thousands of transactions.
No solicitor will readily depart from the scheme it represents. It contemplates that the Vendor will hand over the title deeds and the key of the property in return for the purchase money. The desire of the purchaser to get possession and the desire of the Vendor to get the purchase money is what drives the transaction.
To hand over the key beforehand is to hand over possession. If a purchaser gets possession before parting with the purchase money there is a great temptation to evade or delay the handing over of the purchase money, at least until it suits the purchaser.
Thus, a prior representation that the Vendor will hand over the key before the closing of the transaction will not be complied with. It will be reneged on.
This is not a breach of agreement; the agreement is in the contract. If provision is not made in writing that possession will be furnished prior to the closing, there will be no obligation to do so.