If mail is intended to get there by Christmas, then the Post Office recommends that you post by Wednesday December 20th for second class post, and by Thursday December 21st for first class post.
On average, second class post takes three days, while first class post takes one and a little bit days. Second class post is more reliable over three days than first class post sent at the last minute, which is why these last posting dates are only one day apart. If you are sending a letter somewhere distant like Kent, Cornwall or the Scilly Isles, you might allow a bit longer. Consider posting at the Junction St sorting office in Carlisle for lots of urgent items.