What one should do and what one normally does are not the same. In general we want all central heating supplies to come from the same point, there are a number of reasons, one is so when working on the system you know all feeds are switched off, but also so if you want to supply from a generator, battery etc, again all that needs a feed is fed from the single point.
So much depends on the boiler design, some boilers need a switched line taken from a permanent line in the boiler, so if the boiler fuse ruptures it also makes the switched line dead. However my old oil boiler is simple, not cooling fans run after it is switched off, it is simple feed on and it runs, feed off and it stops.
I know with commercial premises often equipment suppliers ask for isolators fuses overloads etc. But domestic this is normally down to the installer, mainly due to Part P regulations where circuits need notifying and it becomes expensive to DIY due to the notification process. Yes I know we often don't bother, but if the installer has to notify part of the work, then does not look good if there is a clear gap in the process.
I note a 4 pole isolator so could be poles for the control, however "
Equipotential bonding. Electrical connection maintaining various exposed-conductive-parts and extraneous- conductive-parts at substantially the same potential." is a big consideration, this may vary depending on how the whole house is earthed, TT and TN may require a different approach. In essence the boiler will become an outbuilding and
the rules relating to outbuildings must be followed, and since there will be earthed metal work within the boiler one needs to consider loss of PEN with a TN-C-S supply this has been a problem for some time but
an broken PEN it seems is becoming more of a problem.
If your on a TT supply there is unlikely to be a problem, however outbuildings made of metal are presenting problems, we are reasonably safe opening a plastic or wooden door and walking into a house, as we enter the equipotential area through non conducting parts, like the bird on the power cable, as long as everything is same voltage no problem, but once to supply some thing outside then this changes, so most garden tools are class II and have no earth.
So what I am saying getting the supply to an outside boiler is not a DIY job. Here in Wales the garden, kitchen, and bathroom are considered as special areas, and work needs notifying. So really no option. But even in England really the work you are being asked to do is beyond the skills of most DIY'ers.
