As obliged as could be by a mere "scrap of paper". Paraphrasing from Larry Zuckerman who writes this about how the Germans saw Europe come 1870:

August 1914 reaffirmed the adage that when nations prepare for the next war they are haunted by the last one. Before a shot was fired & the ultimatum had been written, the Franco-Prussian War of 1870-1871 had beguiled Germans & Belgians alike. [...] Ever since 1870, German strategists had considered the possibility that Germany's location would eventually involve the nation in a "two-front war" [this can be shown by the Kaiser's increasing "fear of encirclement"]. For twenty years, they had assumed that they should begin that war in a defensive posture, east and west, as 1870 was exceptional & they could not expect lightening to strike again.

[...]

But in 1891, Alfred von Schlieffen became chief of the general staff & he believed replicating 1870 was feasible and essential. [...] When France & Russia signed a military agreement in 1894 [Franco-Russian alliance], Germany's fear of a "two-front war" increased.

[...]

But he [Schlieffen] also believed in the original sin & inherent human weakness, and he spared no one even the kaiser whom he mocking called "willy" in the privacy of his own home. [...] Schlieffen knew railroads which ran like clockwork when properly managed & which drew his eyes westward. In a memorandum he wrote shortly before retirement in 1905, Schlieffen sketched out how the German Army could attack France through Belgium, which had Europe's densest rail system. [This would later be known as The Schlieffen Plan]

Violating the neutrality of Belgium, the Grand Duchy [Luxembourg] & the Netherlands held political, legal & moral implications but this did not concern [Schlieffen]. [To Schlieffen] the violations were "necessary" [...] His logic invoked the doctrine of necessity [known as Kriegsraison] whereby a belligerent can do whatever they see fit to prevail in war [no matter whether it's humanitarian or not]. [Kriegsraison goes against the modern laws of war]

[...]

Kriegsraison's urgency paralleled the feeling in Germany that their enemies had plotted to "encircle" & "destroy" them. Many foreigners treated this suspicion as fantasy. This mind-set viewed attack by Kriegsraison as a means to overcome this anxiety and if the route [to France] went through Belgium then so be it.

[...]

This is how Schlieffen described Belgium in a memorandum from retirement in 1912:

This country is regarded as neutral but in fact it is not. More than thirty years ago, [Belgium] made Liege and Namur into strong fortresses to prevent Germany from invading, but towards France it had left it's frontiers open

Friedrich Von Bernhardi, an German general has this to say about how nations with different levels of power should be treated:

There never have been & never will be universal rights of men

Zuckerman writes that [Bernhardi] goes on to ridicule Belgian neutrality & how Bethmann-Hollweg, German chancellor at the time made a remark about a "scrap of paper".:

Bethmann's remark about a "scrap of paper" was just one example though the debate over this phrase focuses on it's effect not it's content. The effect was immediately noticeable especially in Britain which unlike any other belligerent faced no threat of invasion. "Scrap of paper" justified the war in three plain words and a few British recruiting posters used it as a headline with paper & signatures visible. [see here] This shows that Britain was obliged as any other because that "scrap of paper" bore her signature as Sir Edward Goschen (British Diplomat) states here

TL;DR: GB were as obliged to defend Belgium as any other signatory of the treaty because it bore her signature (as well as the fact the UK were pushing Belgium to become neutral) as Sir Edward Goschen writes, however German Chancellor Bethmann-Hollweg does question why the UK got involved when they were no threat & the treaty was just a "scrap of paper". So it could be said that the UK were forced into war by the ink that had by that time long dried, it could also be said that the treaty wasn't a big enough reason to declare war & that there were other perhaps unknown reasons.

Hopefully this helps :)

Sources & Further Reading

1. Zuckerman. L (2004); The Rape of Belgium: The Untold Story of WWI

2. Solis. D.G. (2006); The Law of Armed Conflict: International Humanitarian Law in War

3. Bernhardi. F (1912); Germany and the Next War

4. Goschen. W (1914); The True Story of a Scrap of Paper

To add to and perhaps modify some of what /u/Subs-man has said, the Treaty of London which was signed in 1839 bore Britain's signature, as it did that of the Netherlands, France, Prussia, Austria, and Russia. It was legally binding on those involved, with Belgium agreeing to maintain complete neutrality and to defend itself against any invader. In return, the signatories would agree to guarantee Belgium's sovereignty and neutrality, and come to its aid if it was attacked. The Treaty was re-affirmed in 1867 following the Luxembourg Crisis and in 1871 following the Franco-Prussian War.

Britain was thus obligated by Treaty to aid Belgium, and the Belgian King Albert I appealed to Britain for support when Germany invaded. Up until this point, public opinion was against entering the war, and the Liberal Party and the Cabinet were split, with the majority favouring neutrality. It was the Invasion of Belgium, more than anything else, that brought Britain into the war; even Churchill, who favoured going to war primarily to support France, considered an invasion of Belgium as among the grounds on which Britain could go to war, even if it was not first and foremost for himself.

• A Scrap of Paper: Breaking and Making International Law during the Great War by Isabel V. Hull