Since 1603 England and Scotland had been ruled by the same monarch. Certain events in the previous few years now meant that both sides thought a proper union would be a good idea. Scotland had a large national debt while England was concerned that any Catholic pretender to the throne might enter the country though Scotland. England agreed to guarantee Scotland free trade while the Scots agreed to uphold the Act of Settlement and that succession would pass to the rulers of Hanover.
The Scottish Parliament was closed and Scotland was given 45 seats in the English House of Commons and 16 lords in the House of Lords. This meant Scotland was not as well represented as Cornwall. The Bill was passed in both parliaments in early 1707 and became an Act on 1 May 1707.
Wales, and indeed Cornwall, had become a part of England by the time of Henry VIII and Scotland now joined that kingdom. As yet Ireland, despite being declared a dependant kingdom in 1541, was not officially part of that kingdom. Successive Tudor monarchs failed to conquer Ireland beyond Dublin. However, 1707, was the beginning of what we now call the United Kingdom.