Ｗithin Great Britain for many years now, there has been a growing movement to make the most of its cultural diversity --to see it as it really is :a nation of different countries held together by a common language and culture.

Ｗithin Great Britain for many years now,
介语短语作地点、时间状语

there has been a growing movement to make the most of its cultural diversity --
存在句式构成主句

to see it as it really is:
不定式短语，与"to make the most of its cultural diversity"可以共同视为主语补足语

a nation of different countries held together by a common language and culture.
名词短语

全句译文：

　　如今，许多年来，在英国兴起了一股日益强劲的风潮，要充分发掘英国文化的多元性——要客观地去认识它：不同的国家，被相同的语言和文化结合起来，共同构成了不列颠民族。

P.S.文中different countries根据上下文判断，可能指英格兰、苏格兰、威尔士、北爱尔兰，这些地区在历史上都曾是独立的王国。