Ruifeng and David,
My tendency is to refer to the flag of the United Kingdom as the British flag, and I think that most of my British friends will agree with me. It is also known as the Union Jack. Please note that this is not the same thing at all as the English flag. They are two different things.
The official description of the Union Jack is as follows:
The Union Flag shall be azure, the Crosses saltire of Saint Andrew and Saint Patrick quarterly per saltire, counter-changed, argent and gules, the latter fimbriated of the second, surmounted by the Cross of Saint George of the third fimbriated as the saltire.
The saltires of Saint George, Saint Andrew, and Saint Patrick represent England, Scotland, and Ireland, respectively, as all of Ireland was part of the kingdom at the time the flag was adopted in 1801. Somehow, Saint David, the patron saint of Wales, got left out.
(Sorry, David, but that's the way the crumpet crumbles. Oh, wow, you've even got a Welsh surname. That's harsh, man.)