Thanks again for your help here.
No, I think it was from a smoothie my sister had made me at hers a few hours earlier. She put spirulina powder in it, and it caused me to have projectile vomiting, which I didn't know was a thing, but apparently people can be violently allergic to it.
I challenged the pcn through Harrow Councils online portal and sorry, I didn't take a screenshot of what I wrote. At that time I was unaware of any signage at all, and I wrote that the notice is invalid without any signs. It was only after my neighbour mentioned the signs much further down, that I knew of them. I mentioned having walked for at least 80m and could see not a single sign on either side of the road. I also mentioned the signpost without a sign.