Hi Rut
My understanding is that all of the wiring harnesses come from the same manufacturer in the UK. I liked being able to get all the other junk you will need from the same place.
There is a special bullet connector crimping tool you can buy from British Wiring, that will lock the bullet connectors in place. It costs $55. If you download the British Wiring catalog you can see it on page 28. I don't know how well it works--probably quite well. The bullet connectors that were actually included in my new harness were crimped and they seem tight.
And, you can also solder them on. That is what I did--I just bought a little butane torch and you stick the wire into the connector--which has a small hole in the end, put a little heat on it and go. If this was a job I thought I would ever do twice I would get the crimper. It would just save a lot of time (and some burned fingers).
Order a dozen extra bullets and also, some new Lucar connectors from BW. You will need them, and that will enable you to start out with clean metal and soft rubber sheaths--50 year old Lucar connectors are just not going to work very well. You can see on your wiring diagram everywhere a Lucar connector is needed--it is represented by a little rectangle.
I also highly recommend buying a professional quality ratchet style terminal crimper for ring and spade connectors, such as the one shown in the catalog. You can get a better price on eBay if you want to go to the trouble. The cheap crimpers are no good and should be outlawed. Treat yourself to a professional quality wire stripper and cutter.
You will need this because your harness will come with a lot of ends which were not equipped with terminals. (I suppose it is because the harness was supposed to be "as original" LOL). And, because there have been changes to components over the years (e.g. I have seen ignition switches and brake light switches with both screw and push on terminals) you will want to get these on right.
I also added dedicated grounds to the rear light fixtures and gauges--you will save yourself a lot of headaches in the years to come by not depending on any ground that is supposed to come through the body of the car (such as the left rear lights.)
I will offer you a couple other bits of advice here... I bought a PVC covered harness instead of the cotton covered. It was a little cheaper. If you are doing a restoration the cotton covered is cool, but if you are going to be adding ground wires to the back lights as I did, it is nice to be able to tape these right to the new harness.
I cannot imagine doing the wiring with the dash in the car. I had it out, getting it recovered with vinyl, and did my wiring on the bench before I stuck it in. I also spent a little time labeling the wires before I actually started to do the job. The green/brown vs. green/red wires vs. green/black vs green/pink wires are hard to tell apart even in the sunshine, let alone on your back looking under the dash.
If they are going to make your harness for you in the UK, one thing you could do is ask (I think the guy at British Wiring is named Josh BTW) if they could put higher gauge wires in for brake lights, turn signal circuits, and running lights and horn. These are of minimal gauge, just as they were originally.
Best,
Charlie