What you were told is officially correct BUT depending where you live there are probably more collections but definitely none on Sunday. My village has a Post Office and I know from watching that our box is emptied at 12noon, 2.30pm, 5.30pm and 6.10pm. I'm told that there's also one at around 8am but the only guaranteed time is the evening 6.10pm.
On Saturday, the only guaranteed time is 11.30am but we have an earlier one, and during the holiday season as we are by the sea, they sometimes collect at about 4pm.
The small boxes on poles have collections at 12noon and the guaranteed one at 5.15pm.
The General Post Office is actually emptied throughout the day from inside as the Sorting Office is connected.
If you live in town your best bet is to post at the General Post Office or The Sorting Office as they have a later guaranteed collection.
Of course, Bank Holidays are NoNo and we once travelled up to London to deliver a Sunday Express Crossword. Didn't win!
www.royalmail.co.uk has a lot of useful information about postal charges, home and abroad with the weights. You can make sure you aren't diddled.
I hope this helps a bit.
Tallulah!
I hope this helps a bit.