Understand that if there is an official reason for Tenerife to be someplace, you will see the Cross of St. Andrew. Scotland would have to use the Union Jack.
Which says more about the reality that Westminster assumes all "official reason" for Scotland , than anything about the St. Andrew's Cross being Scotland's national flag.

And again, the recognized flag of the UK is properly identified as a Union flag, not as a Union Jack. A Union Jack is the jack that is flown on the jackstaff of a Royal Navy vessel. It is a common mistake to refer to the Union flag as a Union Jack, but it is still a mistake.

The Union flag is the national flag of the UK. Period.

Again, using G8 as perfect example of this simple fact, Downing Street has determined that the UK, and not Scotland, is to be showcased during the summit, hence the order that Gleneagles lower the Scottish Saltire and raise the UK's national flag, or Union flag, in its place.

Wikipedia
World Flag Database
Visit Scotland
Scots Independent which makes the following key comments -

"In the latter half of the 20th century, there was a major resurgence in the use of the St Andrew’s Cross, and it has regained its status as the legally established national flag of Scotland. It is used by all Scottish teams in international competitions. It is now widely flown on the flagstaffs of public buildings, sometimes alone and sometimes side by side with the Union Flag. The saltire is also used by many bodies, both private and public, as a logo, a purpose for which it is well suited." [emphasis mine]

Information can be found on this site for becoming a Friend of the Scottish Flag trust, for those that might be interested.

Alba West
Celtia Info
The Heraldry Society of Scotland
World Flags 101
Simply Scottish
Silicon Glen

Each of these links will support the fact that the St. Andrew's Cross is indeed the national flag of Scotland, whether Westminster likes seeing it displayed or not.