Of questionable risk. You can turn off the isolator so you know you are not changing them "under load", and they are single pole made of non-conducting material so there is just about zero risk of a short to anything (unlike the old cast iron cutouts). So that only leaves the risk of personal contact and/or the fuse holder breaking - but we have unswitched FCUs, and most people used to change fuses (before MCBs) without switching the whole house off.One other thing no-one else has yet mentioned - by having those isolators/switches downstream of the corresponding fuses, the fuses could only replaced 'live' (unless there is some way of isolating them upstream of what we have seen) - which would not really be acceptable.
So I'd not be too concerned about that.
As to making the cupboard a locked "enclosure", that has practical problems - and not just by making the means of isolation inaccessible. Unless the "instructed person(s)" is available any time a meter read is needed, then that's a problem.