stock 67 Bonneville
I'm assuming 2-position (on/off) key ignition switch in the sidepanel and rotary lighting switch in the headlamp bucket.
High beam only, no main beam.
For clarity, "High beam" and "main beam" are the same - the one that dazzles oncoming drivers;
the other one is 'low beam' or 'dip (beam)'.
new, made in England, cloth covered wiring harness.
harness has solid blue and blue/white but no blue/red.
So I'm guessing this is really a Wassell "Genuine Lucas"
Indian-made harness that they then shipped to GB and had someone crimp on one bullet so they (Wassell) can take the piss out of "Made In England" ...
"solid brown wire in the headlight bucket coming from the handlebar dip switch/horn button" says the switch is pattern; it should have four wires:-
. one'll be from the horn to the horn button (originally Brown/Black);
. one'll be from Lighting switch to dip switch (originally Blue);
. one'll be from dip switch to headlamp 'low beam' or 'dip beam' (originally Blue/Red);
. one'll be from dip switch to headlamp 'high beam' or 'main beam' (originally Blue/White).
As the headlamp, the Lighting switch and the wires from the handlebar dip switch/horn button are all in the headlamp bucket, nothing is served by Blue or Blue/White wires in the main harness.
When you've worked out which handlebar dip switch/horn button wire is which function, just connect:-
. what should be the dip switch solid Blue wire directly to Lighting switch terminal #3;
. what should be the dip switch Blue/Red wire directly to the Blue/Red(?) into the headlamp bulb;
. what should be the dip switch Blue/White wire directly to the Blue/White(?) into the headlamp bulb;
. if your bike's headlamp bucket has the high beam/main beam warning idiot lamp, connect its Blue/White(?) wire at the same place as the dip switch and headlamp wires connect.
Brake light is working but when lights are on and brake applied the brake light does not become brighter.
If Chris's advice to check for "a stuck on brake switch and ... wires to the brakelight and tail filaments ... exchanged" doesn't find the problem, there is another well-known possibility.
Remove the rear lamp lens and ascertain, when you pull or press a brake lever to actuate the brake light, only
the brighter of the two bulb filaments illuminates.
If it does, turn on lights and ascertain only
the dimmer of the two bulb filaments illuminates.
If it does, with the lights still on, again pull or press a brake lever to actuate the brake light; if the brighter of the two bulb filaments now doesn't
illuminate, the problem is the return path from the bulb to battery +ve (aka "ground") isn't working. This is a possibility if you installed the new harness as part of a restoration that included new frame and/or fender paint,
or the bike has sat around unused for many years?
If new paint, I recommend against scraping any off to expose bare metal ... that'll rust and rust conducts electricity as well as good paint ...
Ideally, run a new (Red) wire from an existing Red wires bullet snap connector under the seat, beside the rear lamp Brown and Brown/Green wires into the lamp and solder the end to the outside of the bulb holder.
However, if it's difficult to run the new wire into the lamp itself beside the two existing wires, I terminate the new Red wire with a 3/16" ID ring terminal and attach it under the nut on one of the rear lamp mounting studs. If you do that, (use your meter to) check there is a return path from the bulb holder to the stud - original Lucas lamp holders are in two parts, they were originally joined electrically by a truly minute piece of braid but, more that half-a-century later, who'd be surprised if the braid had been lost or rusted away?