Right...I used to use British Wiring before it was sold....since have gone with Moss...when I was getting a new harnes for my 1963 Midget, I ordered the Moss harness....it had an extra white wire behind the dash & the Moss tech was researching why...in the interim, I called British Wiring & had him pll one off the shelf in case I needed to get one quick.....it had the same white wire & the number on the bagwas the factory parts number - the same factory parts number that was on the "bag inside a bag" when I got my Moss catalog.
Since I'd had the factory in the UN build me a custom harness for one of my cars, I had their phone number....I called them and the parts number for their harness (that they made) was the same as the parts number on my harness & on British Wiring's harness.....the factory guy told me what the extra wire was for.
Best bet is to find a price you're happy with and a vendor with whome you're comfortable.