This is what MI5 say,
"Threat levels in themselves do not require specific responses from the public. They are a tool for security practitioners working across different sectors of the Critical National Infrastructure (CNI) and the police to use in determining what protective security response may be required.
Vigilance is vital regardless of the current national threat level. It is especially important given the current national threat. Sharing national threat levels with the general public keeps everyone informed. It explains the context for the various security measures (for example airport security or bag searches) which we may encounter in our daily lives."