I don't know which war your talking about so i'm going to add all of them (Most)
1. Great Britain entered World War I on 4 August 1914 when the King declared war after the expiration of an ultimatum to Germany. The official explanation focused on protecting Belgium as a neutral country; the main reason, however, was to prevent a French defeat that would have left Germany in control of Western Europe.
2. The military history of the United Kingdom in World War II covers the Second World War against the Axis powers, starting on 3 September 1939 when Britain and France, followed by Britain's Dominions and Crown colonies, declared war on Nazi Germany in response to the invasion of Poland by Germany.
3. WW3 Belgium's ports were close to the British coast and German control of Belgium would have been seen as a serious threat to Britain. In the end, Britain refused to ignore the events of 4 August 1914, when Germany attacked France through Belgium. Within hours, Britain declared war on Germany.
I added ww1, ww2, and ww3 Because I don't know which one you needed so yea. Might be much but yea.