Dunno what you lot is on about... I regularly see local iso's for IH's, often in the cupboard. The local one means you can isolate easily to work on it.
I don't think that any of us (certainly not me) have said, or even implied, that local isolators are not very common - I'm sure that they
are very common (all my immersions certainly have them, as well as a 'remote' means of control) but what I and others
have said is that they are not really
necessary for things like immersions on dedicated circuits.
The local one means you can isolate easily to work on it.
It does, but the need to 'work of an immersion' usually only arises once every few years, and on those rare occasions, the CU (or the 'remote' switch/isolator, if there is one) is usually no more than a dozen or two steps away.
One thing I would say is that if there is a hard-wired timeswitch, there probably an argument for having an isolator upstream of (and adjacent to) it, since some people might otherwise be tempted to rely on the timeswitch for 'isolation' whilst working on the , with obvious dangers.
Sometimes there is one in the kitchen too.
Yes, albeit less common. To have a 'remote' functional switch (which can also function as an 'isolator') makes a lot of sense if one wishes to routinely control the immersion manually.
Local switches/isolators are probably even more common with showers and cookers, but, since they are on dedicated circuits, the above comments again apply.
Boilers too often have this arrangement.
They do but, firstly, they are commonly not on dedicated circuits (so a separate means of isolation has some advantages) and, secondly, the 'isolator' is usually an FCU, installed because the boiler manufacturer 'required' 3A (or whatever) fusing.
As I have admitted in the past, my own electrical installation is absolutely strewn with 'isolators' of one sort or another (including a good few 'unnecessary FCUs') but very few of them are actually 'necessary', and I would't generally suggest/advise others to follow that example (and I probably wouldn't if I were re-wiring now).
Kind Regards, John