What is an Acceptance?
An acceptance is any words/conduct which objectively indicates that the offeree intends to be bound by the offer’s terms. The offeree’s motives for accepting the contract are not relevant: Williams v Carwardine (1833) 110 ER 590.
If the offeree’s statement does not mirror the offer’s terms, then it is a counter-offer and not an acceptance: Jones v Daniel (1894) 2 Ch 332. This can lead to a ‘battle of the forms’ where different offers and counter-offers are exchanged until there is a match: Butler Machine Tool v Ex-Cell-O Corporation  1 WLR 401.
Asking for clarification or information about the offer is neither an acceptance nor a counter-offer: Stevenson, Jacques & Co v McLean (1880) 5 QBD 346. Clarification and information requests have no special legal effect.
Communication of Acceptance
Acceptance must be communicated to the offeror before it is valid: Entores Ltd v Miles Far East Corporation  EWCA Civ 3. The following rules apply to the communication of acceptances:
Mode of Acceptance
Acceptance can be communicated by words or conduct: Brogden v Metropolitan Co (1877) 2 App Cas 666. Silence without any conduct cannot normally amount to acceptance: Felthouse v Bindley  EWHC CP J35. There are some exceptions to this, however: Re Selectmove  1 WLR 474.
If the offer specifies a mode of acceptance, only compliant acceptances are valid. E.g. if an offer asks for acceptances to be by fax, acceptance by letter will not do: Holwell Securities v Hughes  1 WLR 155. The exception is where the method specified is designed to fulfil a particular purpose, such as speed. In such a case, any equally efficient method will do: Tinn v Hoffman (1873) 29 LT 271.
If the offer states that a signature is needed for acceptance, the courts will normally uphold an acceptance without a signature unless it prejudices the offeror: Reveille Independent LLC v Anotech International (UK) Ltd  EWCA Civ 443. This is because where a prescribed mode of acceptance is meant to be for the benefit of the offeree, they can waive it.
The Postal Rule
An acceptance which is sent by post is deemed to be communicated the moment it is posted through the post box, even if the mail never arrives, arrives late or is only read later: Adams v Lindsell (1818) 106 ER 250; Household Fire & Carriage Accident Insurance v Grant (1878-79) LR 4 Ex D 216. The offer cannot be revoked after posting as a result: Re Imperial Land Company of Marseilles, ex parte Harris (1872) 7 Ch App 587.
Excluding the Postal Rule
The Postal Rule does not apply if the offer explicitly excludes the application of the postal rule: Holwell Securities v Hughes  1 WLR 155. It also does not apply if it is the offeree’s fault that the mail never arrives (such as because the offeree put the wrong address on the envelope): LJ Korbetis v Transgrain Shipping BV  EWHC 1345.
The postal rule does not apply to ‘instantaneous communication’ such as email, telex or answering machine: Brinkibon Ltd v Stahag Stahl GmbH  2 AC 34; Thomas v BPE Solicitors  EWHC 306 (Ch). Instead, acceptance takes place as soon as the message is received, or on the next business day if the message is received outside of business hours.
What is a Unilateral Contract?
A unilateral contract can arise if the offeror makes an offer where he promises to do something if any person does a particular act: Carlill v Carbolic Smoke Ball  1 QB 256. For example, a seriously-intended poster offering £3000 to the first person to walk from London to Newcastle is a unilateral offer.
Accepting a Unilateral Offer
Accepting a unilateral offer does not require communication. Rather, it requires the offeree to do the act requested in the offer: Carlill v Carbolic Smoke Ball  1 QB 256.
Only complete performance constitutes an acceptance. A person who has started performing is not bound by contract to continue: British Steel Corp v Cleveland Bridge and Engineering Co Ltd  1 All ER 504.
Once an offeree begins to perform, the offeror can no longer revoke the offer: Errington v Errington Woods  1 KB 290. This is so unless the context makes it inappropriate to imply into the offer a promise not to revoke: Luxor (Eastbourne) v Cooper  AC 108. The offeror is also not allowed to impede or prevent full performance once the offeree has started: Daulia Ltd v Four Millbank Nominees Ltd  Ch 231.
Does the Offeree Need to be Aware of the Offer?
It is possible that an offeree will engage in conduct which looks like acceptance, without knowing the offer exists. Is it possible for a contract to arise by ‘accident’ like this?
Some reports of Gibbons v Proctor  64 LT 594 suggest that the court decided the offeree does not need to know about the offer to validly accept it. However, other reports suggest that the offeree in that case was aware of the offer when they accepted. If so, any such remarks would be obiter. No other English case has explicitly addressed this issue, though the court in Tinn v Hoffman denied that two ‘cross offers’ (coincidentally identical offers sent in ignorance of the other) could form a valid contract.
In Australia, the courts decided that the offeree must know about the offer at the time of acceptance: R v Clarke (1927) 40 CLR 227.