Yeah fusebox RCBO’s and boards are quite well made and cheap too.Always go RCBO if at all possible. Far less trouble later on and cost difference now is only around £100 in most cases.
Sounds like you have quite a poor installation to start with!
Yes, bonding is important from a shock safety point of view, and on TN-C-S cases also a fire risk as you might have an open-PEN fault trying to divert the neighbourhood's neutral current via some boiler's 0.75mm CPC if no 10mm service pipe bonding!
Basically yes. The typical case for a borrowed neutral is the live of circuit #1 feeds the 2-way switch on floor #1, and in turn to the 2-way on floor #2, and then back via the nearest neutral (circuit #2) and not the neutral associated with the supply (#1).
It is very dangerous as someone might isolate circuit #2 but then someone switches the light on and its neutral becomes live due to the cross-linking.
If you fit all RCBO they will find it fast!
If you have LED or CFL lamps they won't show any conduction at the volt or so used for continuity testing (or resistance testing via a multimeter).
But as above, testing should be done at 250V to avoid damaging a light or dimmer.
No, as in most cases the cross-linking is via some load like a lamp. While you could get the borrowed neutral case appearing in other circuits, and you might get two circuit's neutrals linked by mistake, the most common reason was someone not having 3&E cable for two-way lights on a building with lights split as upper floor on one circuit and lower floor on another, etc.
I have seen the stopcock for the water pipe and seems like it’ll be a nightmare to get to. The gas is just on the other side of the consumer unit apparently fingers crossed.
Okay the borrowed neutral thing makes sense. So you know how I mentioned about the continuity method and you mentioned that I may still get continuity through led lamp... hypothetically if the lamps were removed or switches turned off then I did the continuity test between 1 neutral and another neutral it should be clear right?