Many post offices have collections through the day, here one at 12.30 and again about 5pm. Does a 5pm collection make any difference to a letter or parcel i send first class? Is it more likely to get there next day with the 12.30 collection even though they should in theory be the same?

Doubt it makes much difference, first class items picked up at 5pm should still be on the overnight transport to wherever they're going.

The 12.30 collection just goes to the mail centre ready for when the sorting shift arrives so they have mail to start before the main bulk of mail arrives in doesn't make any difference, just helps with volume of mail later on

Isn't the 12.30 collection a hangover of postal services in London? I know they had something like 6 deliveries per day so you could send a letter in the morning, have a response later in the same day, and then send another reply. I know there also used to be more than 2 collections from post boxes (at least in Liverpool) until maybe the past 10 years or so.

I also guess logistically it makes a lot more sense. Rather than have the handling and sorting offices overwhelmed by receiving all the mail at once, it's paced out during the day and could even possibly start it's ground journey slightly earlier.

Busier postboxes also need to be emptied more than once or they'd be overflowing by 5pm.