I purchased a wiring loom from Advance Auto Wiring in the USA some time ago with a view to fitting it to the car when I did get around to doing the rebuild. It's a really well built kit, the wiring gauge is way thicker than a standard loom and relays and connections are incredibly robust and well thought out… however, the loom is not terminated at all and just has bare wires on the ends. This would not have been an issue for me if I had been doing all the wiring myself as it's just my time… but when you're paying someone else to do the job, it's very time-consuming to terminate every wire and the cost and time for doing this would be quite significant. Additionally, I had specified various 'enhancements' to the car which would involve a lot more than the standard wiring and relays. Also, the only location that I could mount the distribution block in the car would be behind the glovebox, which is also where the ECU was going to be mounted for the Electronic Fuel Injection (more on this later).
For all the above reasons, I decided to approach Autosparks in Nottingham (https://www.autosparks.co.uk/) to make me up a custom loom. They can add various additional circuits to a standard TR6 loom such as ancillaries (in case you might get some insane notion to fit heated seats to the car for instance), hazard switch wiring and electric radiator fan. Note that the main loom and rear loom are also sold separately. They were incredibly helpful and produced the loom as an urgent order for me within a few days since we only decided not to use the Advance Auto Wire loom after the bodyshell was back on the chassis.
Since I had so many additional fusing requirements, we ditched the original Lucas fused distribution box in favour of a more modern one which gave us more options.
Mainly for audio reasons, I swapped out the standard battery for a Deep Cycle one, which also had the additional benefit of dual-terminals, so I could run and fuse all the car electrics off one pair of battery posts, and the audio off its own pair of terminals.