Even if they breach the Building Act and knock down all the walls in the flat without approval. If they knock holes through the floor then this may create dangerous conditions and controls may be necessary using Article 31 or the Building Act (if it is caught in time).
How would you know that they have knocked down walls or put holes in ceilings without carrying out some kind of inspection or assessment ?
This is absolutely why it is necessary that assessors and housing association look at all areas of the blocks.
If you are a housing provider and you are going to have a stay put policy in your block of flats you need to know that the existing fire precautions will support that policy.
You say yourself Kurnal if someone plonks a hole in a floor of their flat compromising the FR you may need to carry out measures under article 31
Yes it is a ball ache, yes it can get political, but tenants and owner occs need to be given an ultimatum, and you have to be honest with them and explain that in order to guarantee their safety you assess all of the block including their flat to do an assessment.
If you have problems with individuals then get the fire authority involved, and if an owner is completely obstinate you might just have to note you couldnt gain access stating the reasons why, and if and why you were unable to take any legal action against them (highly unlikely).
Remember that there are often service shafts in a block of flats located seperately from the traditional communal area - it might be that the only way to reach that shaft is via an access panel in someone's flat. Because it would be a common shaft, it would be a communal area, and fire has the potential to spread throught the block via teh shaft so you need to check it isn't compromised.
I doubt very much that the owner occupiers can prevent entry - yes they might refuse - but the freeholder has a right to inspect any components that makes up the freehold of the building at a reasonable time. You may have to issue letters, threaten with court etc or may not be able to get in straight away, but I dont believe any freeholder can be refused entry.