Any responsible business owner, who is considering engaging the services of your company, will want to know that all of your staff are properly trained and licensed by the SIA and preferably that your business is properly accredited under the SIA's Approved Contractor Scheme. They will also want to be sure that you hold public liability insurance, employers' liability insurance and professional indemnity insurance. Once you've got all that lot sorted out, you can then start marketing your company.
See here for loads of useful info and advice:
https://www.simplybusiness.co.uk/knowledge/articles/2019/04/how-to-start-a-security-company-uk/