If you're going to quote t&cs on me, you need to understand the context

if you have a Britpass, when you get it validated at a station, you must give them the date you want it to start (I had a consecutive one, so not sure about others). On the ticket, the ticket office enters (
inserts) the start and end dates. The above quote is referring to the fact, if like me you choose to start you Britpass on a sleeper, the next day's date must be inserted. I confirmed this with CS before travelling and was certainly not trying to fair dodge one bit.
Edit: I realise not many people start a consecutive Britpass on a sleeper, and the above t&cs are contradictory if taken literally and most likely apply to the non-consecutive Britpass. This is why I confirmed it when I reserved my seat with CS.