The alert system explained on Wiki:

The BIKINI state was an alert state indicator previously used by the UK Ministry of Defence to warn of non-specific forms of threat, including civil disorder, terrorism or war. Signs giving the current alert state were displayed at the entrance to government buildings and military installations. It was established on 19 May 1970. According to the Ministry of Defence, the wordbikini was randomly selected by a computer.^[1]

Whilst similar to the DEFCON alert states used in the United States, the BIKINI levels were defined by the section of the military or organisation rather than UK-wide, and as a result, countermeasures and reactions to differing states may differ as acutely as from building to building. The highest levels of alert, RED and AMBER, were only intended to be maintained for limited times. The WHITE state has never been used in the history of the system.

It was replaced by a more general and public terrorism alert status, UK Threat Levels, an alert state system in use by the British government since 1 August 2006.

• Red = Information has been received about an attack on a specific target. It can also mean “red alert”, meaning the United Kingdom is at war, especially if there’s a likelihood of a nuclear strike .
• Amber = There has been specific information received and there is a substantial threat to government targets. It can also mean “high alert”, which could be a transition to war.
• Black Special = There is an increased likelihood of an attack, but no defined target. It can also mean “potential terrorist threat”.
• Black = There has been an assessment made that there is the possibility of an attack, but no defined target. It can also mean “possible civil unrest”, meaning safety can’t be guaranteed.
• White = No information available about a specific threat. It can also mean “situation stable”