Almost: the important point is they have no right to impose any fines; any punishment will be done by the British Transport Police in accordance with British law, and there are relatively few crimes of possession in the UK (drugs are an obvious one, but even then a first arrest for Class C drugs is typically dealt with by means of a police caution). Essentially: any punishment arising from a security checkpoint would be a police matter, and have nothing to do with those manning the security checkpoint.