Ok, that is good useful info, so let us try to make a start. The horn, hazards, headlamp flash and interior light "should" be fed from the purple circuit, which is fed by a fuse from the brown permanently live circuit. So, we have an inconsistency, because hazards work. However, that probably tells us that one or more of the brown wires is adrift somewhere like my initial guess, but not the one feeding the purple fuse, and also a purple wire is adrift somewhere. Put a new bulb in the interior light and see what happens, as that will provide more useful information.
Fog light, for legal reasons, is wired to not work until headlights are on (although that may not be true if they have been bodged), so it may or may not have a problem of its own, depending on what else is found, so that one can be left till later, once the main lights are all ok. (Actually, long after your Mini was made, the law was changed so that you can now drive in fog, etc, with front fogs, side and tail lights, without headlights at your discretion, and in that condition rear fogs may also be used, as long as they go off when there are no headlights or front fogs, but your car will not be wired that way, although we can discuss that later, when everything else is fixed. I just put that in for completeness, in case some smart alec takes me to task for being allegedly incorrect.)
Indicators working says that wiring from the switch to each light, and their earths, is intact, as is the flasher. That is good, one less problem. There have been lots of indicator problems recently.
Leave the tacho for now, it may resume working when other things are fixed, but if not you will by then have more information to go on. But in case you have a spare moment, the tacho signal comes from the coil negative via a white-black wire, and there will be another of the same going to the distributor, or the engine will not run. Coil positive should have a single white wire, for unballasted ignition, or white-pink and white-yellow if ballasted, but you never said anything about engine problems, so perhaps I am needlessly digressing. There is also a black earth, a lighting feed (usualy red) and an ignition-controlled 12V supply (white, or more likely green via a fuse), but the details depend slightly on which tacho you have.
So the way forward can be to initially examine all brown and all purple wiring (I think there is some purple-orange too, with its own fuse from purple, but I don't have my wiring diagrams right now), which hopefully are still the standard colours. I am very reluctant to suggest purchasing a new loom, and definitely not making one at home (need hundreds of pounds worth of good crimp tools to do a reliable job), so hopefully you will find some simple faults, most likely corrosion at terminals. One thing at a time, and you will get there.
