Many years ago, when I was a teen, I lived in a village/small town with a post box near our house away from the main street. There were two collections a day, one in the morning and one in the afternoon. (Over time those collections reduced.)
In the bigger town we live in now, 30ish years later, there is only one collection a day from whichever post box you use. The posties drive around from estate to estate and empty the boxes as they go, so they are all emptied at different times of day, but generally in the same order.
We were in a local city about three weeks ago for a meeting and needed to post a letter to do with the meeting afterwards. Went to the nearby post box, on the main street and found it was only emptied once a day and we'd already missed the collection as it was done in the mornings. The letter box was on the corner of the main street and the road that came off the roundabout there had the post office on it. (Yes, we still have a Post Office there!) You would think the busiest post boxes, especially those in the vicinity of the Post Office, would be emptied again close to the end of the working day and at least dropped into the PO overnight.
Our letter never got delivered either so it nearly cost us a decent percentage of our monthly income for this month and we would have to start an application from scratch again. Thankfully we sorted it out before we lost it.
Also, Royal Mail do not guarantee delivery of first class post by the next day, they have not guaranteed this for many years. If you need to send something for a certain date it needs posted in plenty of time if possible.