If a potential passenger wishes to transit from landside to airside at a UK airport, they must reasonably expect to be have their belongings and person searched. If, when they arrive at the security area, the potential passenger decides not to be searched, then that is their right. Security staff at arches and x-ray machines have NO legal right to demand to search them or their baggage or to detain, arrest or anything else that has been suggested in this thread. Of course, if the potential passenger refuses to be searched, then security staff are absolutely correct to refuse entry airside. They presumably would then ask the potential passenger why they have tried to travel in the first place and if the security staff have grounds to suspect that the potential passenger has something to hide, they could, if they wish, call a police officer.
At the moment, (subject to proposed imminent change) Police officers at aiports have no rights to " stop and search" on suspicion. Of course, someone who has arrived at an airport and refuses to be searched and still tries to gain airside entry would probably give a UK police officer strong grounds to believe an offence is likely to be committed so in reality, they would be dealt with quite firmly.
To the question about insulin pumps or any that may arise about false legs, pacemakers etc, most owners of such devices know exactly how to present themselves to staff and invariably do so.
As an aside, these security staff, both visible to passengers and at aiport worker security stations, are subject to constant checking by independent organisations which pass themselves off as bona fide travellers or airport workers. They are tested on their search and examination techniques and have a pass rate of over 98%.
The role security staff carry out is mandated by government and abhored by passengers and flight crew alike. Despite this, they do the best they can under difficult circumstances to protect all of us who fly.
If they get it right, nothing is said, if they get it wrong, they will live with it for the rest of their lives.
I think we owe them a debt of gratitude.