It won't start if you can't close the boot. Try it with the key card in the slot.
I've found a list of the panels needed to replace the floor, perhaps put a price list together and see how it compares with the garage/insurance estimates-...
Technically harnesses have to be at least 3 point and have a BS marking (British Standard) to pass an mot. Some testers might be picky but as long as you can prove they conform you'll be fine.