I suspect I know the answer but...
I'm looking to use something like one of these...
https://www.tlc-direct.co.uk/Products/GLSPIKE50BK.html?source=adwords&kw=&gclid=..... fitted with a 5W LED to illuminate a house sign.
The route from garage to sign (8M) would be underground so I went looking for some SWA cable. I discovered that 1.5mm core seems to be the smallest (?) and that has an OD of >12mm.
The lights I've seen are typically supplied with just 3m of rubber cable which has a 7mm diameter. So the supplied gland won't take 12mm SWA and anyway 12mm SWA would be stiff making it hard to adjust the position of the light and have it stay in position.
So it looks like it would need a length of 1.5mm SWA, an encapsulated junction box and the lamp with it's rubber cable. All seems overkill for a 20mA draw.
Alternatives?
PS I'm aware of the requirements of Part P - qualified electrician etc