How should I connect wiring for this ceiling light?
Thanks for all your help folks. With a little help from a multimeter I was able to confirm that the smaller brown was indeed switched-live, and the two with the sleeves indeed neutral and live. I've just terminated the live and wired the switched-live and neutral to my new fitting. I guess the intention with the live is, as some people have suggested, for ceiling fan, or might also come in handy for smart home stuff like Philips Hue lightbulbs that always need power.
I've selected Ed Beal's answer because it contains the solution I found.

If only 1 switch cap 1 of the hots and see if the other is hot with the power on swap hots as needed with only 1 switch and cap or cover the other for safety