The (approximate) UK equivalent to USA romex is our "twin and earth" (T&E) cable.
In the typical UK case the CPC (earth/ground wire) is uninsulated within the overall cable's sheath, however, it the CPC should be sleeved in green/yellow at the ends where it connects to the sockets, etc. Also here the CPC is a size or two below the line/neutral conductors (e.g. 2.5mm^2 L&N conductors with 1.5mm^2 CPC).
The reduced CPC size, along with its unsheathed construction, makes the cable overall smaller and easier to install, also cheaper. However, we have quite a strong requirement on the design side here in the UK to make sure the over-current protection disconnects fast enough not only to prevent the CPC overheating, but also to avoid prolonged exposure to high touch voltages during fault clearance.
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In mainland UK most domestic work is in T&E and conduit is not so common. Many decades ago singles in metal conduit was used routinely for domestic and you will sometimes find T&E pulled through old conduit when rewired (instead of the singles that would be the norm in industrial style conduit installations).