I would get the light first, and see if you can fit a trouser box inside the base. IF you can, SWA. If not, 4mm split con, preferably ducted. 25mm black civil duct is great for this - the coils of it, not rigid.
Good news 1: The lights have arrived in time for me to work out how I'll connect them.
Good news 2: There's room inside the bollards for a 20mm galv Y box, so I can loop SWA in and out of those.
I assume that the threads are compatible, and I'll be able to screw 20mm SWA glands and a 20mm nylon compression gland into the entry points of the conduit box?
Good news 3: The box will be above ground inside an IP44 light, so I won't need any special waterproofing measures.
The last light in the chain is surface mounted on a wall, with a rear cable entry point so it won't take a SWA gland, but as I don't need to maintain the armour continuity beyond that point I'll just use a compression one.