It's a strange one, at first glance it looks like it used to be an intermediate switch, but how long have intermediate switches been around? did they even exist in the days of green earth sleeving?
Given that there are wires left unterminated, I sure hope (whatever it was) that it was disconnected at the other end.
Agree with the second paragraph!
The first paragraph, it is intriguing, we have a roughly similar setup in our bedroom, one switch at the door and one at the bed, and the switch at the bed has the lamp connected. I used 3c+e to connect them.
I think the blue would be neutral and the red and yellow are the switched lives, either one is live depending on the position of the main switch., The switch at the lamp end would switch the light between those two.
There may have been a second lamp that was daisy chained, and they used 3c to chain it for some reason? Lack of 2c cable perhaps.
Hopefully that makes sense.
To the op, you would need to remove any other blanking plates in likely places and look behind the light switches etc.
You can probably get detectors to follow the cable across the wall if you're struggling.
And when you work it out, assuming it is 1.5mm² cable or more you can connect a new socket with a fused connection at the other end, so no need to dig up your wall. But it might be less hassle to just dig the cables out and drop new ones in as an unfused spur.
