Single point of isolation?

I have heard people say there should be a single point of isolation, then I look at my new install, one isolator turns off whole house except for two double RCD sockets and a FCU supplying the central heating.

And also there are two DC isolators for the solar, a AC isolator for the inverter output, a new consumer unit not a clue what it is for except connected with solar panals, and on the inverter a DC switch. So if I want to replace the FCU with a RCD FCU for the central heating how do I isolate it? The DC switch can not be reached due to a battery being in front of it. The FCU clearly marked EPS supply, but as yet only the compliance certificate has arrived, oddly before the work was completed.

OK I have worked live before, but not really what one should do.