Kurnal,
Sounds like a can of worms, however not one that is uncommon in areas with local acts. I have come across several similar examples.
You asked about building control regularisation, the new owners could apply for a regularisation to ‘regularise the works’ however you have stated that it is fully compliant with the B regs so not much point. If it was un-compliant then the local authority has a 12 month window from ‘offence’ to enforce. Beyond the 12 month the only remedy is an injunction. Which can be taken out at any time after the contravention if the offence is serious enough, i.e. life threatening. However this method of enforcement is very rarely used. Anyway your client complies with the B regs so no need to worry.
In terms of local acts it is worth noting that they are generally enforced by the local authority and not the fire authority. Most Local Acts ask for monitored fire alarm, 100% perimeter access, suppression and roof ventilation when storage is above 7000m3… all very nice if it was consistently applied across the country. However it is not and that is why it is wrong. Many fire authorities recognise this and in many instances take a more pragmatic approach.
I am aware of several instances where Fire Authorities have agreed not to request the enforcement of the LA by the Local Authority (in storage buildings with no suppression or ventilation) on the condition they are satisfied that 1) ‘normal’ ADB type fire safety provisions are in place and people can escape safely and in time before untenable conditions occur 2) an assessment of the fire load, time to untenable conditions and time to flashover has been calculated and made available to them.
With this information the fire authority are able to establish the best course of action for tackling a fire at that premises. I.e. not to enter. I am also aware that in these instances the Fire Authority has made it very clear to the building owner that they won't enter the building in the event of a substantial fire. (Expected)
The owners are usually satisfied as they haven’t had to spend the money. If a fire does occur the Fire Authority (and the RA) can say we told you so.
Maybe you could try this approach for your client?