Normally, you pay the 10% deposit the day before exchange of contracts. This is paid to your solicitor and not directly to the seller. Once the money is in, your solicitor can then proceed to exchange contracts.
Sometimes, the seller will ask for a pre contract deposit to take property off the market. This is meant to lock you into the purchase, as the deposit is non refundable. This would typically be paid when your offer is accepted, and you must ensure it is part payment of the purchase price and not an additional fee paid to your estate agent.