Min deposit to owner is generally 3% ( can be agreed to differently, more or less ) on signing of initial S&P, balance to be paid in 2 weeks ( from memory ). At any stage after signing of the initial S&P if either party backs down from the deal they have to pay back the deposit plus 100% penalty, also as a rule the Agent will want his/her full fee ( 1% ) on top.
Pulling out of a 8.5M deal in the first two weeks should cost the buyer 3% of 8.5M = 255,000 plus 100% plus agents fees, after that initial period it becomes a hell of a lot more costly.
Owners can elect to except the full deposit ( 5 or 10% ) straight up which makes it far more costly to default in the early stages.