https://www.quickwire.co.uk/product/quickwire-switch-load-junction-box/
If there is enough slack on the cables you could use one of these and then you would only have one cable going to new fitting
Put the light on the wall so that the cable entry point is level with a mortar joint either bottom or side and scrape out some mortar so the cable sits inside enough for the fitting to sit flush
First thing you need to do is buy a fire extinguisher, One suitable for electrical fires, Second thing STOP watching tutorials and third thing is get an electrician
Also the double socket will not be rated for 40a, you may have to use a switched fused spur to feed the socket, so 6mm to sfcu then 2.5mm to double socket