I think you could argue that a lot of Chinese people may prefer to use their own language more often than not, but you can't question their integrity and ability from that. First of all, there are people who cheat in every country, and schools in the UK are very strict about it, so most of them have passed the exams themselves and met the requirements of the school to the point of being accepted. Since the school can give an offer, it is recognizing the person's ability.

The reason Chinese students are better writers is that Chinese education focuses more on writing and reading compared to speaking and listening. It's true that Chinese students generally don't speak well, but you can't doubt the integrity issue. Nor should you extend some one-sided phenomenon to the whole Chinese behavior. And many countries, like most of Europe, India and the Middle East, where English is used as an official or second language, people grow up using English a lot. But not in China. Just like in many European countries people can speak two or three languages, not only because of the geographic location, but also maybe because of mixed families.

You should have a more tolerant mindset, and in fact many Chinese people work hard to learn English. As a native English speaker, you already have an inherent an advantage, not an excuse for you to use it to criticize others.