[Madhatter]

At the very least change the fuse to a 3 amp. most security lights are 500w and 3 amp goes up to 690 watt. There's lots of things can go wrong with a security light, or it can get vandalised or sabotaged, if it shorts out with a 13 amp fuse, the wiring will melt and may catch fire before the fuse blows. The main reason for not using grey cable (which is also available in white) is that its not flexible. When a plugs pulled out, the cable bends and eventually will break, but still connect, which will cause arcing and could cause a fire. The cable bits up to you to decide, do you unplug it very often.
Not relevant to your situation but household grey cable for fixed wiring has also changed it colour coding, It used to be red-live Black-Nuetral Green/yellow -earth, It's changed to Brown-live blue-nuetral Green/yellow-earth. Grey Black and Brown are live on three phase 415volt supplies, they used to be red blue and yellow. Also worth pointing out is that that grey cable shouldnt be used as extensions to connect christmas lights outside, It can go brittle over time in freezing temperatures, you should use blue coloured arctic flex for christmas lights.