You are responsible for providing 10% on sigining-this is standard in any contract of this type. If you don't have this in cash, you will have to borrow. The bank will not let you draw down the mortage until nearer the time the property is 'sold' (you need to have all insurances in place, survey etc).
In some cases, the bank will give you a bridging loan. In others, you will have to find the money some other way (savings, another bank, Credit Union, family member). If you can't come up with 10%, ask your solicitor if the vendors are willing to accept a smaller deposit.
When you receive the mortgage funds, you then repay the 10% loan.