You need a pass to go into the part of the hall where the votes are actually being counted and laid out on the tables. All parties for that count have a number allocated to them and usually choose close supporters and/or family. It is the task of those people to watch the votes and point out when ones for your side are in the wrong pile. For the actual announcement, usually in another hall and away from the actual voting papers, other people can be present and, at all counts I have been at, the Police are visible. The postal vote is crucial in most constituencies these days although, personally, I believe you should only get it in very exceptional circumstances and it should have heavy safeguards on it to protect democracy.