OK, here's my 2 cents worth. I'm going to go out on a limb here, and firmly join the "non-Lucas" camp regarding wiring in PV's. I've rebuilt at least 6 or 7 over the last 20 years, including complete rewiring. I think that given the other electrical components of the PV being German; (Bosch for the ancilliaries, VDO for the instruments, and I'll assume for the dash switches, too), the Prince of Darkness didn't get his hooves into the PV series. The spade type connectors, together with the ceramic fuses is, for me, pure 50's and 60's German engineering (look to VW Beetles). I've scrounged the foot dimmmer switches out of older Mercedes in the boneyards (German Hella), as well as the dome light covers (also Hella).
It's the English pedigree of the P1800 that dictates the Lucas wiring.
The 1800 wiring loom has the typical British color coding, which is different from the PV and 120 series. The glass type Lucas fuses in the 1800 should seal the argument.