By Erle C.J. in Offord v Davies (1862) 12 CBNS 748 at p.757:
This promise by itself creates no obligation. It is in effect conditioned to be binding if the plaintiff acts upon it, either to the benefit of the defendants, or to the detriment of himself. But, until the condition has been at least in part fulfilled, the defendant has a power of revoking it.
Byrne St Co v Van Tienhoven St Co (1880) 5 CPD 344, Common Pleas Division per Lindley J:
It may be taken as now settled that where an offer is made and accepted by letters sent through the post, the contract is completed the moment the letter accepting the offer is posted… the writer of the offer has expressly or impliedly assented to treat an answer to him by a letter duly posted as a sufficient acceptance and notification to himself, or, in other words, he has made the post office his agent to receive the acceptance and notification of it. But this principle appears to me to be inapplicable to the case of the withdrawal of an offer…
It appears to me that both legal principles and practical convenience require that a person, who has accepted an offer not known to him to have been revoked, shall be in a position safely to act upon the footing that the offer and acceptance constitute a contract binding on both parties.