Quick response: I did this sort of thing when I worked for both central and local government. It’s an at least semi-specialist area although there’s much more guidance and advice available this days. Start with National Counter Terrorism Security Office web site ProtectUK | Home and maybe contact your local resilience forum. It’s very dependent on what you do and where you’re doing it. Also why someone might wish you harm.(or anyone else nearby).

 If you’re working from scratch maybe start with a discussion group about how what your organisation would do with a telephone bomb threat and work upwards from there. That gets more people involved and thinking about it. This is also handy Bomb threat checklist | ProtectUK.

 I would definitely not plunge a site straight into a full bomb drill until staff have been made aware you’re assessing and planning for this and have a degree of confidence they can be protected.

Also be realsitci about the time it takes to prepare and validate plans for this kind of thing.

...and of course when(if?) the  Terrorism (Protection of Premises) Draft Bill becomes law you might be required to come up with a terrorism plan and staff training depending on what business you are in.

It might be a good idea to get in touch with the CTSO now rather than wait until the legislation is passed as I suspect they will be very busy once it is finally adopted.

Back in the day when we were scared of Irish accents it was all about evacuation out of the building and even away from the site (starburst-dispersal with no hanging around at assembly points which are a great place to have your secondary device). Nowadays invacuation or lockdown is as important. Many recent terrorist attacks involve individuals armed with knives rampaging across the site. In those circumstances locking and barricading doors makes more sense than assembling everybody in one easy to find spot. The tricky bit is how do you communicate this to people on the ground. A Tannoy is good but who has those. There are apps with messaging functions but then of course you want people to be using their phones.  

Mobile phones are prone to giving away potential victims in the scenario of a rampaging attacker.

Government alerts, pre-programmed alarms, app notifications, friends & relatives calling/messaging because of the live streamed unfolding situation, key pad tones, haptic feed-back / vibration, battery level warning, start-up / shut down sounds along with the clatter of a dropped hand set.

Ideally they should be switched off until the situation can be considered safe.

Originally Posted by: A Kurdziel

How else will people know it’s safe?

The government shouldn't actually know who is in an area to telephone or message them.

Then not everyone has a phone and for those who do not every phone is charged.

You go old school with the emergency responders doing a sweep of the area and if necessary using a bull-horn to get attention.