The original 500 limit was, in retrospect, a psychologically astute accommodation targeting beginners who feared the very idea of not eating anything for nearly an entire day. The "two days a week" and "you can eat it tomorrow" also relieved culturally-induced hyper-concerns of starving. These subtle changes made the 5:2 approach palatable to a very large audience.
In the original video, if the good doctor would've extended his multi-day fast for only a day or two longer, he would have experienced what many others have since discovered - the point where hunger "just disappears". The original book would likely have been very different.
The new book:
https://www.amazon.co.uk/Fast-800-combi ... el+MoseleyAlso:
https://thefast800.com/From the interesting kindle excerpt at Amazon.uk, the 800 is an adjustment with the idea that it could also be used as an every day limit for months at a time. But that requires calorie counting, doesn't it? Granted there are a fair number of useful things every faster should know before they start, but consider that an every other day full fast - that's zero calories - might accomplish the same thing. The devil's in the details.
The Kindle sample contains ample evidence of useful and necessary updates, so I'll look for the book when it becomes available in the US.