Until exchange either side can walk away without legal penalty — the source of gazumping and gazundering. At exchange the deal is locked: if the buyer then fails to complete they typically forfeit the deposit and can be sued for losses; if the seller pulls out they face a claim.
The deposit is usually 10% of the price, though a reduced deposit can be negotiated. Risk in the property passes to the buyer at exchange, so buildings insurance should be in place from that date, not completion. The gap between exchange and completion is commonly one to four weeks but can be same-day.
Scotland works differently — missives are concluded by the solicitors and bind earlier in the process. In the UAE the binding point is the signed Memorandum of Understanding (Form F) with the deposit, not an "exchange".