Right guys, found this out a few months ago.
If you go in to a post office on a Friday and ask to send a parcel special delivery Saturday guaranteed it would cost you £10.74 to send a small parcel.
But if you use royal mails online postage it only costs up to £8.55, £7.25 if its under 400g.
Now I've done this 6 times since and everytimr it's got there the next day.
So if you send a lot of parcels and use royal mail specifically which I do as it covers me for up to £500, then this is a nice saving.