O.P said he had to submit the share certs to the broker so this could not relate to a nominee account, my experience has always been that the broker would not do the trade without being in possession of the share cert so I would say they are relying on your goodwill to provide it which in my view you should anyway as presumably you were satisfied with the price on the day you placed the order to sell.
Remember the sale has crystallised, the share cert is needed to transfer ownership of the amount of shares you sold on the day to the new buyer, it is a real and tangible transaction.