The official last posting date for Christmas delivery (for 1st Class mail) was yesterday but I'd still give it a reasonable chance of getting there on time if:
(a) it's posted early tomorrow ;
(b) it's posted at a sorting office, rather than in a post box or Post Office ; and
(c) it's going to an address which isn't too far from a main sorting office.
(e.g. deliveries to major cities might be OK but don't hold out too much hope for the Scottish islands!)
If you use Parcelforce's Express 24 or Special Delivery services, tomorrow is the official last posting day. If you use Royal Mail's Special Delivery service the last posting day is Friday.
Chris