Certification has dramatically different levels of substance behind them and you should be aware of these to ensure you remain attentive to your fire safety obligations when employing external contractors to help fulfil your duties.
Then you get into the levels of Certification. For this, people should be looking for UKAS Accredited Third Party Certification. Whilst this is commonly referred to as Third Party Certification it is important to know the difference between UKAS Accredited and non-UKAS Accredited Third Party Certification.
Third Party Certification is always for a specific service, this in most instances will never cover all services a provider may offer. Providers may have achieved UKAS Accredited Third Party Certification for multiple services, but each service offered will have had to undergo independent assessment to provide evidence of their competency for that specific area.
Further Reading:
UKAS - Accreditation Vs Certification
https://www.ukas.com/accreditation/about/accreditation-vs-certification