...What's the value @ £800 per year?
The value of an insurance system is peace of mind: Hopefully, nothing will happen and you lose all your investment but at least if something goes wrong, insurance will cover it.
In case you haven't noticed, the repairs done by Tesla arevery expensive.
You can fix your built-in charger for about 20£ fuse but Tesla would swap the whole charger for thousands of pounds.
Same if your heater is broken.
Same if you need to swap out the memory card of your 17" screen computer: Tesla would swap the whole computer out instead of just the memory itself.
The list goes on.
Some items to break down in old age:
Air conditioner
Model S door handles
Headlight (Tesla doesn't change the light only, it's the whole thing that costs almost 1,000£)
GPS antenna...
If you are able to fix a Tesla, there's no need for the insurance.
Otherwise, if something goes wrong and you go to Tesla, it'll be very expensive.