The only building regulations that apply when making an insurance claim are those that were in effect at the time of the works. Many houses have open plan areas, such as three story staircases, which were perfectly acceptable at the time they were built but would not be allowed if built now.
Building regulations are always reactive - they are adapted and added to when a certain practise or technique would have protected life or property had it been used in the past. The fact that you are required to install adequate fire/escape protection is only because hundreds of familes have been devastated in the past by not having it. Be thankful that they made that sacrifice to keep your family safe, rather than trying to find a way to ignore it.
Are you saying that all houses where stud walls have been taken down (and there are a lot) have made their house insurance void?
Yes, they have. Or more specifically for any fire damage or associated loss of life their insurance will likely be void if the wall was required by the building regulations, and indeed they could be pursued for significant costs and face criminal charges if any third parties are affected. After a fire the insurance company and their loss adjusters will check for any building work that could have affected the loss, and will check to ensure the building regulations compliance of that work. They will know which building regulations were in force at the time of the work because of the LA notification and compliance certification so they will know if a wall should be there. If it wasn't then they will adjust the cover accordingly, and if the non-compliance was a cause of any/all damage or loss of life then they will remove all cover for that claim.
Fire doors should always remain closed when not in use, and are usually fitted with closers to enforce this. It is allowable to hold a door open as long as it automatically released when a local fire detection alarm is activated, but it must not be propped open. Properly fitted fire doors will self-seal into the frame and can give 30 mins minimum and sometimes more time to enact an escape or be rescued. Crucially they block smoke passage which is usually the killer in domestic fires. A stud wall will offer similar protection.
Fortunately I have never personally had to escape a building that was on fire. However I have been back into one afterwards (fire in a flat) and the striking image that left with me was how effective fire doors are at preventing the spread of flames and smoke. The direct damage and smoke staining inside the flat was severe, yet standing on the other side of the main fire door you would never know of the damage.
You don't mention the details of your house/conversion, but I'm assuming that fire doors have been requested for the occupied rooms that open onto the existing stairway, or that the new stairway be enclosed with a fire door? Fire doors don't have to be ugly, most styles can be made to FD30 spec. Normally a stud wall would only be required to create a protected escape route (e.g. stairs or a hallway), so you may be able to change the layout to make it less obtrusive.
