(The Union Jack wasn’t actually adopted to Great Britain until New Years Day in 1801)
Parliament and Queen Anne, a daughter of James II who became Queen in 1702, helped to unify Scotland and England. There had been three previous attempts to unify them back in 1606, 1667, and 1689 but each time one or the other country discouraged this idea. So, with the help of the monarchy, the two countries found they both had some good reasons to unify and they would resolve their problems and work out differences as a whole country. Here are two reasons why they unified.
Scotland wanted to gain access to colonial markets in New England after their colony on the isthmus of Panama – the Colony of Caledonia – failed very badly.
England decided that if the rulers stayed the same – because Scotland was faithfully Protestant and England was Anglican – they could prevent any religious civil wars with each other.
But this new country needed a new name – a good name, a strong name – and they decided on Great Britain. The countries were unified in 1706 and 1707, after years of being separate and not always being kind to each other. They got over their differences and become one unified country.