I wondered about the cassette leads myself. This was an ebay purchase. They guy said he was the original owner. But who knows, it's ebay afterall
I definitely recognized them as the same ones that came with the ZX80. And since this 81 is definitely an issue 3 it seems unlikely that these cassette leads are original. I took it apart because it had a dead keyboard. So when I replaced it with one of RWAP's amazing replacements (thanks Rich!) I noticed the board issue number.
Everything else about it seems in order though. This box is very similar to the one I received with my ZX81 and even my original ZX80. Both came in these plain unmarked cardboard boxes. Were the containers in the UK fairly different for the 80/81s?