Go to evening college, i hope you like maths, you won't get an apprenticeship but don't worry, you won't need one, someone will employ you. Good luck and Good choice :)
You have the idea already, just run the 3core & E from your existing switch to the other, shouldn't need JB's, wire this cable the same at each end, and your existing cable to the L1 and L2.
I'm not aware of any installation cables with just black cores, so i guess your friend was mistaken, it could be that they have sleeved or cut off all the reds or browns, which is very stupid and would certainly contravene wiring regulations, yes.
