With lighting cables at 1mm they are rated at 10.5 amp Reference Method 101 (above a plasterboard ceiling covered by thermal insulation exceeding 100 mm in thickness) so in real terms not a problem as the ceiling rose is used as a junction box and that is only rated at 6 amp so we never use a MCB rated over 6 amp or at least we shouldn't. And using LED lighting you are really talking about very few amps my upstairs lamps in total are under 230W in fact more like 60W and at 230W it uses 1 amp so really not a problem.
If you were really worried then using maintenance free junction boxes there would be no problem extending.
However power cables is another story. Although maintenance free works great with 5A at 45A used with for example a shower they are really no good and any joint has a risk of poor connections and as a result fire and the cable is so easy to over load. 16mm cable is still only rated at 46A Reference Method 101 (above a plasterboard ceiling covered by thermal insulation exceeding 100 mm in thickness) and 6 mm cable is down to 34 amp.
The normal method to extend power cables is to add a device. For a shower at 45A then adding an isolator is a good method to allow you to extend the cable.
What I would consider is with LED lights it is often better to have the lights spread around the room, clearly easier to add extra lamps before the installation goes in.
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