Unless you have been living under a rock you will know that we are in the second wave of the CV-19 pandemic. As such and compounded by the time of year, Royal Mail is under extreme pressure and mail is taking longer to arrive than usual. I am even starting to see Tracked 24 packages sit in my local depot for 24hrs+ before moving on to local depots. As these packages were sent with other Tracked 24 that have moved on, I am assuming Royal Mail is prioritising those where the delivery depot is running at good capacity and holding back those where local depots are affected by closure/short staffing (just an assumption).
I highly recommend ordering early (and I mean NOW) if you want delivery before Christmas and to be honest I also recommend Special Delivery if practical – Royal Mail seems to still be prioritising SD mail.