Probably a bit late to the party here, however one route you might look at is in Australia we have vehicle engineers, but we have a component of the law for Individually Constructed Vehicles.
Now, depending on the engineer you speak to, and these are generally blokes at Fabrication Shops, Registered Mechanics, etc, they might be able to work the modifications you've done to be considered less a "Conversion" and more a "New Build"
So that may
be a loophole in the UK as well (A lot of our laws are the same, like some bored bureaucrat decided to just Ctrl + C and Ctrl + V the legislation.
Essentially your rego papers wouldn't say "Mazda RX8" anymore, but "Dave's Coupe" or whatever you want to call it.
Though if you would like a guide if the UK doesn't have regulations on electric car conversions yet, then you could have a look at Vehicles Standards Bulletin 14
under sections NCOP13 Section LV Alternative Power Units V2
and NCOP14 Guidelines Electric Drive V2
which are what we have to meet in Australia to ensure safety, and that is not just for conversions, that's for new builds on things like Kit Cars.
The other thing that might feel like you have wasted your time on though is that in Australia (and maybe the UK) is that if it is a Kit Car/Low Volume, at the Federal Level at least, you only need Seat Belts as your safety gear, no airbags, no traction control, no stability control, none of that.
Though I have watched all of your videos, and they have been a gem to me (Especially since you started pulling the leaf apart, I have a leaf myself (e-NV200) and I'll be using that for my Jetta build) and you seem to have heavily invested time and mental power on the CANBUS issues. In fact that has been quite a guide for me in my understanding of the process, and I'll be looking at something similar for mine, though as I'm not keeping things like the factory gearbox, I'll be needing more CAN data relayed.
Anyways, best of luck to you mate, just some food for thought in that maybe you "don't own an RX8" anymore.