Почему флаг Англии величается Union Jack? В 2-3 предложениях.

In fact Union Jack is a sailing term. The correct name for the flag of the United Kingdom is Union Flag. The Union bit refers to the union between Scotland and England that was effected in 1707.
In the navy the flag is called Jack, hence the name Union Jack. The reason the latter became common usage probably stems from the newspaper reports during the Napoleonic Wars when the Royal Navy won most of the engagements against the French. When a French ship was captured, it would strike its own flag and the British would raise their flag over the French and sail it into a British port where the papers would gloat over another ship coming in with the "Union Jack over the Tricouleur".