Under the terms of the treaty known as the Entente Cordiale (1904) the size of the British Force to be sent to France in the event of hostilities breaking out was set.

Undermanned:
Naturally the British Force was undermanned in comparison to its continental equivalent, the British force comprised entirely of volunteers the larger European Armies were conscripted.

Unprepared:
The British Army were taught some pretty painful lessons in South Africa, but learn them they did. By 1914 when it came to infantrymen skilled in "fire-and-manoeuvre" tactics they were second to none, and although small in number they were perfectly capable of making their presence felt. According to the Historian George Gordon - " The BEF was probably the best trained and most experienced of the European armies of 1914"

Ever heard of something called the "Mad Minute"? It was an individual firing drill. The British Army training of the day emphasised rapid marksmanship and the average British soldier was able to hit a man-sized target fifteen times a minute, at a range of 300 yards with his rifle firing over 36 rounds per minute. This ability to generate a high volume of accurate rifle-fire astounded German troops who had to face it and it ended up playing an important role in the BEF's battles of 1914.

Battle of Mons 23rd August 1914:
German Forces - 160,000 men supported by 600 guns
German casualties – 5,000 men
British Forces - 80,000 men supported by 300 guns
British casualties – 1,638 men
Result of the engagement was that even although outnumbered two-to-one in terms of manpower and artillery the German advance was successfully and crucially delayed. At the end of the engagement the BEF far from being destroyed was still a force in being.

Does that sound like humiliating defeat?

Battle of Le Cateau 26th August 1914. - Another successful holding action halted the German advance for a further five days, the BEF still continued to exist, it continued to manoeuvre and it was ready in all respects to play its part in the Battle of the Marne which stopped the German Schlieffen Plan in its tracks.

First battle of the Marne 5th – 12th September 1914 - Decisive Allied Victory
Allied Forces – 1,071,000 men (39 French Divisions + 6 British Divisions)
German Forces – 1,485,000 men (27 German Divisions)
Allied Losses – 81,700
German Losses – 220,000

All over by Christmas - Was a popular misconception held by all sides, it was not the official view held by any of the combatant powers. The popular misconception was fueled by recent experience of wars involving industrial powers dating back to the Franco-Prussian War all of which were of short duration so to the general public why should this one be any different.