Royal Mail are reporting delays in deliveries due to COVID-19 and sheer amount of parcels coming through the postal system. Therefore they are advising everyone to order anything for delivery as soon as possible.
All our post is sent by a tracked and/or signed for method (depending on which country it is going to), and they say that they’re prioritising this type of mail, but even so I’d order sooner rather than later if you’re ordering Christmas presents.