And don't think shortfalls that weren't picked up by BCO's & subsequently completion certified are safe from further action - a completion cert does not in itself mean FSO compliance.
Recent example - Premises required compartment walls between occupancies & escape tunnel. Got relaxation from Secretary of State as long as tunnel provided and 3m walls used instead of full ceiling height (over 12m). Built tunnel, but didn't bother with walls, but still got completion cert.
A couple of years later FRS inspect after complaint and issue notice and take steps to prosecute because of insufficient compartments/limitation of fire/smoke spread, no fire alarm, etc.
Completion certificate deemed by FRS as more use as toilet paper than a defence to the notice.
If going into fire, it's worth gaining some knowledge of the old codes & why they were used and how to recognise older type precautions such as doors, alarms, lighting, etc & how they work as most of our buildings in this country are quite old and a lot of older kit is still in use and this has thrown some dual trained H&S/fire staff who have done a course only showing current technology and current standards, with them missing or wrongly interpreting things at 'real world' sites.