I just start changing at 00:30 and mostly that works for me. Where it does run beyond 04:30, I’m only paying 14p KWh, just 9p more than the cheap rate, and that is still way cheaper than public or super charging.
Why there isn't a simple start and stop time nobody knows .. and maybe it will change in a software update. (There are a number or reasons I can think of but not to have start and stop as one of the timer options does come as a surprise.)
Its the same reason there are no waypoints. Tesla/Elon think they know best. They want to keep things simple so only include the minimum options and the ones they chose are the ones that make sense to them in their environment / use case. If you look at the manual in the description of "scheduled departure" it says "NOTE: Your vehicle aims to finish charging before 6am (end of off-peak hours)" So they acknowledge the concept of off peak but base it around some fictitious fixed US concept of it and tough to everyone else.