MI5 Terror Alerts by E-mail
Sounds like security theater to me:
But he added that one of the difficult questions was what people should do about the information when they receive it: "There's not necessarily that much information on the website about how you should act and how you should respond other than being vigilant and calling a hotline if you see anything suspicious."The first, called Threat Level Only, will inform the recipient if the nationwide terror threat level changes. The condition is currently listed as severe.
The second more inclusive service is called What's New, and will be a digest of the latest information from MI5, including speeches made by the director general and links to relevant websites.
I've written about terror threat alerts in the UK before.