Royal Mail say for 2nd class that they aim to deliver your mail by the third working day after posting. Aiming and hitting the target are very different as men using the toilet will know.
For christmas posting 2nd class, the last day was 18th Dec
I don't want to depress you, but Parcel Force accepted a parcel for me on 3 Dec and their tracking system gives an expected delivery date of 7 Dec. I'm still waiting.
...well I'm not sure about 'orders', but I posted all our Christmas cards to catch the 5-30pm collection last Thursday and the local ones were delivered the next morning! I rarely use 1st Class post...I think the Royal mail is an excellent service...and I really mean that!