So, rather than making 3 phase AND twin and external wire / armour
earth they use 3 phase for both and only need to make . stock one type
...?

Pretty much, it's a supply and demand thing. If they only made half as
much of each colour combination it would theoretically cost twice as
much.
You can get allsorts of cables and accessories for electrical, (and
plumbing), that get you out of tricky situations but they're
overpriced and little used so no-one stocks them. So it's the make-do
this way method then!
...

SJW
A.C.S. Ltd.