This is not a Royal Mail stamp - first clue, no Queen silhouette. You can buy these at tourist places where postcards are sold, and you can pop them into a Royal Mail post box. However, it may take weeks or months for your card to arrive (if at all).
The flag here is the St Andrew's Cross, also known as the Saltire. It is one of the oldest national flags still in use though it's origins are shrouded in myth and legend. In 1385 the Parliament of Scotland agreed that the white cross on a blue background should be worn as an identifying mark by Scottish soldiers (contrasting nicely with the red and white of the English St George's Cross), but it had been in use possibly as early as 832. One of the legends goes that in that year, in East Lothian, there was a battle between the Scots and the Anglo-Saxons. Before the battle, King Angus of Scotland saw a vision of clouds in the shape of a diagonal cross. After winning the battle, the sign became the flag of Scotland, a white cross in a bright blue sky.
for Sunday Stamps - flags