The decision to lower the international threat assessment level from 'Severe' (meaning an attack is highly likely) to 'Substantial' (meaning an attack is likely) was made by the Joint Terrorism Analysis Centre (JTAC). This is an independent body of experts who evaluate available intelligence alongside terrorist capability and intentions.
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. The international threat level has been at either 'Severe' or 'Critical' since 29 August 2014.
The official statement from UK Protect can be found here.
You can find out more about how the threat levels are set, what they mean and when they have changed by visiting the Security Service MI5 website.