is the sentence grammatically correct?

correct，亦清楚。
要睇清楚成句結構，先簡化一下：
While China's rise may be inevitable, Britain's success was more surprising.
句意清楚：「（奧運獎牌榜上）中國冒起不可避免，但英國的成就郤令人訝異。」
而 given it has one fifth of the world's population to choose from 呢截，代名詞 it 跟住講 China o既一句，佢代表o既就係 China，咁呢句就係補充 China o既一ｄ事實：「考慮到她有全球五份一人口來挑選（運動員）」。冇問題喎
圖片參考：http://tw.yimg.com/i/tw/ugc/rte/smiley_4.gif

Your sentence is awaked....perhaps because it is out of context. You may consider the following revision in order to make it more concise (as a single sentence).

While China's rise (as an economical powerhouse, as the leader of gymnastic sports, as an international leader, as.....) my be inevitable, given it has one fifth of the world's population to choose from, Britain's success was more surprising (in comparison).

If you can define China's rise "to what/as what" may be inevitable, the sentence will be clearer to the reader as a single, stand alone sentence. In addition, I would end the whole thought with "in comparison", because it ends your sentence nicely with a retrospective review of the fact that you're trying to compare China and Britain's growth.