I'm jumping in here to hopefully clarify:

"13:00 December 25 2025 Europe/London" is local time but its Unix timestamp is an estimate depending on how the tzdata database looks right now.

"13:00 December 25 2025 +0100" is also local time but it is immutable.

Using the latter can only be done reliably for present and past (but it conveniently avoids looking up the offset later). All future times must use the former to be stable otherwise you should probably be using UTC.