Main dipped headlights not working

Are you referring to the foot operated (dimmer switch) or the dash mounted (headlight switch)

You need to learn to call these by the terminology in the service manual.

The two feeds to the dash mount HEADLIGHT SWITCH are completely separate. "That switch" is in actuality SEVERAL switches in one "box"

There are AT LEAST three separate switches in the light switch, that is, isolated "poles."

One outfeeds park lamps ONLY in the park position on older cars. I have no idea what the Brits did

One outfeeds tail and dash lamps in both the park position and the head position. On later cars this contact also feeds park lamps

The outfeed above "splits" and feeds the dash lamps into the twist / turn dash lamp dimmer control

Last there is the COMPLETELY SEPARATE switch for ONLY the headlights. This infeeds via the large gauge black wire, and outfeeds via the green to the foot dimmer switch.

So with the headlight switch pulled all the way out BOTH the large black and the green should have full battery voltage.