Answer:
In order to apply for an SIA license in UK, you need to obtain SIA training. SIA training courses such as Security Guard, Door Supervisor, CCTV are offered by a number of companies such as Linx Security Training; http://www.linxtraining.co.uk.
The license is valid for three years, upon successful completion of the training course, you can apply for the license. SIA application packs can be ordered through the SIA website.