The thing is, Great Britain never promised to protect Belgium. The Treaty of London only guaranteed Belgium neutrality by the powers that signed it. It means that they would never attempt to attack Belgium or draw her into a war that she did not want. read more
The Treaty of London of 1839, also called the First Treaty of London, the Convention of 1839, the Treaty of Separation, the Quintuple Treaty of 1839, or the Treaty of the XXIV articles, was a treaty signed on 19 April 1839 between the Concert of Europe, the United Kingdom of the Netherlands and the Kingdom of Belgium. read more
Obviously the UK wanted to protect the throne in Belgium. Hence the Treaty in 1839. When Germany invaded Belgium in WW1, this contravened the Treaty and this is why Britain defended Belgium. As a Belgian living in England, I am grateful. read more