The Building Regs look after the design of the building but have little to say about the management of the building during its lifetime. The RR(FS)O covers the lot.
Also, sometimes new builds do not meet the requirements of the RR(FS)O because of poor design and incompetent supervision by the Building Control body. Often, in such cases, there has been discussion between the b.c.b and the FRS but it has been too little, too late and has been inconclusive. The lead authority, who controls such discussions, is the b.c.b. so it is usually their responsibility to ensure that proper consultation takes place.
If you're interested in the consultation process between b.c.b.s and FRSs read "Building Regulations and Fire Safety: Procedural Guidance." This is freely available on the world wide interweb net.
Stu