Most of Willesden is very expensive and posh. If you're thinking of
Willesden Junction, that is actually in Harlesden.

There is no postcode for Harlesden and NW10 is officially "Willesden"
though it covers also Harlesden, Park Royal and Brent Park / Neasden.
And parts of Willesden Green are in NW2 which is Cricklewood.

So it's difficult to draw a boundary and say "that is Willesden" unless
you use the NW10 postcode area.

Postcodes are alphabetical between NW2 and NW10. I don't know how
Golders Green gets to be NW11 though or what else it might stand for,
so a separate postcode for Harlesden would have to be NW3½ (to fall
between Hampstead and Hendon).

After that we hit the disused railway
and just take our road along it.

That railway isn't disused.

Don't know what it's used for, but if it's just for freight they might
be better placed in lorries on a road. Clearly there are no passenger
trains along that line (and if there were would there be enough
passengers? After all, where does it link to? Hendon Thameslink?)