For design, installation, commissioning and/or ongoing maintenance of a fire detection and alarm system, look for a company BAFE registered to the Fire Detection and Fire Alarm Systems (SP203-1) scheme. The SP203 schemes are modular, so ensure they are certified in the area you require. As mentioned above, these areas are: system design, installation, commissioning and maintenance.
More information can be found here: https://www.bafe.org.uk/do-you-require-fire-detection-and-alarm-system-works
Legal obligations nationwide require the appointed responsible person for fire safety for commercial/non-domestic premises to have adequate fire protection.