This is where a Quickwire Switch & Load comes to the rescue. It comes equipped with a loop in, loop out, switch and a load connection. This means you can simply disconnect the old ceiling rose and reconnect the existing cables into the Quickwire junction box. For the load output, simply connect an additional piece of T&E and wire this into the light fitting.
It only takes a few minutes, and best of all a Switch & Load is so tiny it can be pushed up into the ceiling void without having to widen the existing hole more than 35mm. It also conforms to BS 5733-MF, making it a maintenance-free junction box. For a bit more insight, here’s a great video from Gaz at GSH Electrical: