Why are lights flickering, but only in one room?
It's possible there's a bad connection between the fixture and the supply wiring, I've had situations where I screwed down a chocolate block (nickname for a screw-down terminal block in the UK) as much as I could but there was still a bad connection in it. I've since started using Wago splice connectors as they are much more reliable, and very quick to use.
However, this sounds like bad wiring to me, either the wiring supplying the switch or from the switch to the fixture has issues. This could be the wires themselves being faulty, perhaps the insulation is breaking down, or the gauge of the wire can't deliver the power needed for the fixture. This is potentially dangerous as it could cause a fire, so I would not mess around. Try replacing choc-blocks with better splice connectors, if that doesn't work it's probably best to get a pro in to track down the source of the issue and replace bad wiring.