Bear in mind though that in the usual conveyancing process, once you exchange contracts on an agreed completion date, that date is legally binding and there will be penalties under the contract for any late or lack of completion which can be significant (from daily interest rate to loss of deposit and more in certain circumstances).
So, you need to make sure that everything is ok'd and in place so that there is no problem with all monies being in place (cleared in your law firm's account) in sufficient time for completion from you, your lender etc...
It is usual to pay a deposit on completion, it's the seller's security. Obviously, if you exchange and complete on the same day then it's pretty negligible as the whole amount will be paid across in any event. It's very rare for a deposit not to be required.