First, call the Pakistan Embassy at 44832525 and tell them about the loss. They are closed this Sunday and as such call them on Monday between 8 AM and 1 PM. They will guide you on the steps to be taken to get hold of a new passport. The embassy will provide you with a new passport, but it may have you doing some running around such as going to the police and making a report of the loss, placing an ad in the newspaper regarding the loss etc. However, the issue would soon get solved. Don't worry.