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Also, never connect all the same colours together in a lighting circuit, just throwing that in there for later because it's a classic, we've all done it and it never works :)
Been there done that - fitting a new kitchen light in Cycliste's flat in Huddersfield many years ago. Blew every RCD, circuit breaker and fuse in the entire block. At which point I discovered it was time to leave to catch my train back down to London. Bad Brommers.
Brommers
You might expect a floating earth wire in those circumstances.
It should have some kind of termination on it just to ensure it doesn't end up poking in a line or neutral terminal somewhere. Tape would be 'ok' if earth colour coded. Sometimes they are just clipped right back in lighting circuits.
Insulation resistance problems usually only occur when the damp or water has had a chance to break down the insulation on the copper wires. You then get arcing between the conductors. If you didn't have an RCD this would eventually heat up and then it's easier to detect!
It looks like the first picture is your lighting loop in and out with a switch line (this is an educated guess there are other possibilities). If you can identify and mark them and note the connections in the white box you could disconnect them and try and diagnose the problem from there.
Do make sure you use a voltage tester and work safely. If you don't feel confident or know what safe working methods are in this case you might want to consult a professional. If you don't have a voltage tester or continuity meter you might already be at your limits.
Also, never connect all the same colours together in a lighting circuit, just throwing that in there for later because it's a classic, we've all done it and it never works :)