Excellent comment. The topic has become an illustration of the disparity in the competence of not just Electricians but across the board of trades and professions. The rules are written by professionals/experts (sometimes irritatingly) mostly to prevent catastrophe including lost lives with...
...it's been quite the learning curve from apprenticeship, lead sheathed, (sheath bits made for an excellent mouse !) vulcanized, single core(s) 2 core without earth MICC et al, Conduit an and hallelujah PVC conduit, dry lining boxes, weaning from Wylex and MK products, today range of option...
Good news , your post howled of reversed polarity from the get-go, a registered Sparky to finally oversee, what amounts a DIY, well worth the cost. The RCD gadget from eBay, wasn't waste of money after all, alerted you to a dangerous situation. The "know all's" quick to write of anything that...
...indeed it is john2713 the only thing missing, it wasn't wired in wet rope, tis a sad day when a noble plasterer shakes his head, at such "work" but then all trades are often/sometimes, all judged by this kind of stuff.
Me too also, if I see an earth tag on a metal box I use it if possible, call it unnecessary habit, tradition, whatever you like but don't say don't bother.