Since Mar 28, 2005
The United Kingdom is made of four constituent parts (see flags below) of England, Scotland, Wales and Northern Ireland.
Great Britain is made up of England, Scotland and Wales.
Hence the official name of the "United Kingdom of Great Britain and Northern Ireland"
Great Britain was formed in 1707 with the "Act of Union 1707" in which Scotland and England (with Wales) dissolved their own Parliaments and ceased to exist as independent nation-states. Great Britain was then formed.
Scotland is not England, nor is England part of Scotland. Wales is not England etc. All consider themselves separate nations existing in one nation-state of the United Kingdom.
To this day Scotland and England both have separate legal systems, education systems and national Churches.
However, all the nations are joined by a shared and interdependent history with each other including love, hatred, distrust and friendship.
(In fact it is a lot more complicated than that but I don't want your head to explode. So a Scotsman is not English but is British. An Englishman is not Scottish but is British. A Welshman is neither Scottish nor English but is British, A Northern irishman can be British, Irish or Northern Irish (or two or three!) so we all have totally different nationalities and yet exactly the same!!!)
So if you mean England say England, but if you mean the UK - DON'T SAY ENGLAND!
On a related topic, and while I have your attention. Scotland and Wales are not English colonies; they are as much equal members of the UK as England and they can vote for independence (as could England) but they don't. England just happens to be larger that's all - 85% of Britons are English. The English are a surprisingly tolerant people and are truly the greatest large nation on earth. Scotland, Wales and Northern Ireland are the best small nations, so what a family eh? :) Although I must admit pray England don't win the Soccer World Cup - We would never hear the last of it! ;)
If you want more information start here: Click Here For United Kingdom - wilkipedia
The flag of the United Kingdom
The flag of Scotland (St Andrew's Saltire)
The flag of England (St George's Cross)
The flag of Wales (Red Dragon)
The unofficial flag of Northern Ireland. (Ceased in 1973, now the Union Flag of the UK)