Contact your local police via their 101 number with as much detail as possible about the person and give the person's name and address and the address of the garage. They will then pass the information onto the relevant department. Also seek advice from the police on how to warn other garage owners and neighbours as to any possible danger.
The Police will then consider what action to take. Reporting is very important as intelligence gathering and evidence will need to be collated.
You can also report it to the Anti-Terrorist Hotline on 0800 789 321.
If you feel that the situation is an emergency then you should always dial 999.