Royal Mail are advising the following last UK posting dates this year:

Standard (48HR Tracked) - Friday 20th December
Express (24HR Tracked) - Saturday 21st December
Special Delivery Guaranteed By 1pm - Monday 23rd December

Please note that whilst these are the last recommended posting dates you would still need to place your order well in advance to ensure we dispatch in line with their advice.  Any order could still be liable to a delay within the postal service that is beyond our control.