The basic setup with the headlights is quite a chain (bracketed numbers are Haynes diagram keys):
ignition switch (03500)
external lights switch (03054)
steering column dip/main switch (04020)
fuse for each filament on each side of the car.
then to the earth block you've discovered behind the (UK) passenger side headlight.
To get consistant and understandable results, you need to use a single good earth point for your meter, and measure everything with respect to that.
The battery -ve is the best place.
Although you have cleaned the earthing point, it will still be worth measuring the voltage at that point on the individual connectors.
You might be surprised.
If that's not the problem, and the bulb seating isn't an issue either, you may have cooked one of the three switches detailed above, as they (and the wiring) are only just up to the standard 40/45 Watt bulbs.
IMHO, the only way to get the brightness you'd expect, and some life from the switches when using uprated bulbs, is to feed them from pair of relays, which in turn need to be fed from a substantial (and fused) supply.