What time do you need to have a parcel in by to meet the last drop off for next day delivery - first class?

I want to send a parcel tomorrow, from Scotland to England, to get there for Saturday.

It's quite a small parcel, not very heavy. When I sent one once at around 2pm I was told I missed the last drop off time for first class next day, but it would get there the day after tomorrow.

What time should I have it in by tomorrow?What time do you need to have a parcel in by to meet the last drop off for next day delivery - first class?
Do it as early as possible. Just as the last daytime train of the day to London has to leave Scotland around 5PM, so parcel collections have to leave similarly early. If there are two collections, some items in the second collection might not make it for next day delivery.What time do you need to have a parcel in by to meet the last drop off for next day delivery - first class?
as soon as it opens to be sure. if it were the other way round its before 4