There are millions of foreigners in China and if China relaxed its immigration regulations, there would be millions more from the poorer countries. In addition, there are hundreds of thousands of foreign students in Chinese universities. However, China is not an immigrant society and I do think they need to relax some of their requirements and regulations in order to attract more foreign talent.It'll be a great injustice to compare China with Britain.
Britain is a fully developed, super high-tech, advanced nation.
China is a developing nation. It has yet to be fully developed.
Yes, people get impressed with everything new and shiny that China puts up these days but this is what Britain was putting up decades or even a century ago. So, what will China's current infrastructure look like in a few decades? Will it still be shiny as today?
The most important thing which is yet to be seen is that by the time China is fully developed and Chinese people are well off, will the country remain unified as it is today? I believe China still has many societal changes, experimentations yet to go through.
Also, despite China looks new and shiny, billions of people from the developing world dream of migrating to the Britain. Nobody wants to go to China. Why?
As far as why immigration to the UK has historically been more desirable, it is pretty simple, because of the dominance of the British empire. The Uk was the metropole so naturally it attracted many of the citizens from its former colonies. However the UK today is mostly running on legacy and these impressions are more due to the past rather than the future or the rapidly changing reality of today.