If you ever get very far out of town you see Romex used for everything.
It's the same this side of the pond. Many DIYers don't seem to know that anything else exists, and you'll see our equivalent of NM used for everything. The "high quality" installs for sheds, greenhouses, and so on may actually have the cable pulled through a length of old garden hose, although quite probably with a taped join in the middle where a couple of odd lengths were dug out the back of the garden to be pressed into electrical service. The slightly lower quality jobs (
) just have the cabled buried directly -- Sometimes as much as 12 inches below the vegetable patch.....
For proper underground junctions, we have these plastic boxes which can be used to joint our SWA cable:
http://www.tlc-direct.co.uk/Products/PRUBX13.htmlOr for junctions which will not need future access, we can use a resin-filled joint, although "code" requires the splices within to be crimped in this case:
http://www.tlc-direct.co.uk/Products/SWJK1.html