For Scots living in areas under Level 3 or Level 4 pandemic restrictions, it remains perfectly legal to fly to an overseas vacation—but illegal to go to the airport to take off.
The strange situation arises because of the devolved powers of the United Kingdom's nations, and the non-devolved powers held by the central government.
Scotland, Wales, and to a lesser extent Northern Ireland have made decisions of their own on Covid-era rules. Scotland has at points and recently taken a harder line on travel, asking people in Level 3 and Level 4 areas to stay in their own areas and avoid unnecessary travel, and has urged the UK to limit leisure are travel.
So, the present situation, as explained to the Scottish Parliament by one of its ministers: “International travel won’t be illegal because it would be for the UK government to do that, but travelling to the airport or to a port for that purpose would be against the regulations.” A new law has gone into effect enabling violators to be fined.