In uk.d-i-y, Christian McArdle wrote:

I think it is actually safer than splitting the circuits. Throw one MCB and
you've turned off the entire heating system and associated cabling. OK, so
you should throw a DP isolator as well, but many don't bother and you don't
want to electrocute these people just because of their ignorance.

Certainly seems reasonable to me too, especially with halfway decent
labelling - "immersion + CH controls/pump". All I meant to point out
in the original context was that there might be unrelated and mildly
surprising appliances/sockets sharing one MCB in any installation
you've acquired through home ownership (rather than laying out for
yourself from scratch).

Stefek