As you've been told, they wouldn't improve safety, so are really not worth installing (only advantage is that they can easily be 'reset', avoiding the need to 're-wire a fuse') but they are pretty easily available - as per this from TLC ...Thanks for the reply. As you say fitting circuit breakers to the old box adds nothing and they appear to be unavailable anyway.
It would make absolutely no sense to fit a 100 mA RCD. If you feel that you must, at least for the time being, put off replacing the CU with a modern one (with all the modern devices in it), in order to avoid the need for notification, then for goodness sake at least put a 30 mA RCD up-frontSo thinking of fitting an upfront 100 amp 100 mA RCB at this time and then maybe swap over the CU later with MCB's and 30 mA RCB in the box and maybe a surge protector as well if I can run to that.
Kind Regards, John