Usually yes - just the DNO main fuses before it. Once installed, there shouldn't be any reason to isolate it.
However Ryefield are only one of several manufacturers of these things, and there are other styles such as this Lucy one which have the DNO incoming fuses within the same cabinet behind a separate door:
Building owners, and they are also the ones who pay for it to be supplied and installed. However once fitted that's usually it - these things are designed to last for decades so maintenance/replacement is somewhere between rare and never.
Before - the outgoing circuits go to a meter for each flat/apartment, and then to the individual consumer unit.
Yes, that's what they are designed for. They have fully enclosed fuseholders with cartridge fuses inside, and the holes that the fuse blades fit into are also shielded so that the exposed blades are already disconnected by the time they are visible, minimising any arc that may form there. Basically the same type of fuse and holder as the individual 100A cutouts.
Unlike that MEM board which has none of those things.
Yes, although obviously the proper protective equipment must be worn and the correct procedures used.
In better installations there is a separate cutout within each flat with a solid link rather than a fuse (in a red carrier) so isolation can be done there.
The MEM installation looks as if someone attempted to do the initial install on the cheap - Ryefield style cabinets have been available pretty much for ever, and that MEM board is the wrong thing to be using there, even when it was new.
