Union Flag

"The Union Flag shall be azure, the Crosses saltires of St Andrew and St Patrick, quarterly per saltire counterchanged argent and gules: the latter fimbriated of the second, surmounted by the Cross of St George of the third fimbriated as the saltire."

• Flags.net A comprehensive listing of all world flags, current and historical
  
• MrFlag.com | A Short History of the Union Flag
  
• MrFlag.com | The Union Flag: A Historical Perspective
  
• MrFlag.com | The Union Flag: It's Status as the de Facto National flag of the UK
  

Additional

This is just flags in general, but I couldn't pass it up;

• BBC SPORT | Flags guide a guide to the various Grand Prix flags.